Restore LIBPNG for png files

Restores the use of libpng for png files as stb does not support writing out 16bit png files.

Engine/lib/lpng/tests/pngimage-full

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+#!/bin/sh
+exec ./pngimage --exhaustive --list-combos --log "${srcdir}/contrib/pngsuite/"*.png

Engine/lib/lpng/tests/pngimage-quick

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+#!/bin/sh
+exec ./pngimage --list-combos --log "${srcdir}/contrib/pngsuite/"*.png

Engine/lib/lpng/tests/pngstest

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+#!/bin/sh
+#
+# Usage:
+#
+#  tests/pngstest gamma alpha
+#
+# Run ./pngstest on the PNG files in $srcdir/contrib/testpngs which have the
+# given gamma and opacity:
+#
+#  gamma: one of; linear, 1.8, sRGB, none.
+#  alpha: one of; opaque, tRNS, alpha, none.  'none' is equivalent to !alpha
+#
+# NOTE: the temporary files pngstest generates have the base name gamma-alpha to
+# avoid issues with make -j
+#
+gamma="$1"
+shift
+alpha="$1"
+shift
+args=
+LC_ALL="C" # fix glob sort order to ASCII:
+for f in "${srcdir}/contrib/testpngs/"*.png
+do
+   g=
+   case "$f" in
+      *-linear[.-]*)
+         test "$gamma" = "linear" && g="$f";;
+
+      *-sRGB[.-]*)
+         test "$gamma" = "sRGB" && g="$f";;
+
+      *-1.8[.-]*)
+         test "$gamma" = "1.8" && g="$f";;
+
+      *)
+         test "$gamma" = "none" && g="$f";;
+   esac
+
+   case "$g" in
+      "")
+         :;;
+
+      *-alpha[-.]*)
+         test "$alpha" = "alpha" && args="$args $g";;
+
+      *-tRNS[-.]*)
+         test "$alpha" = "tRNS" -o "$alpha" = "none" && args="$args $g";;
+
+      *)
+         test "$alpha" = "opaque" -o "$alpha" = "none" && args="$args $g";;
+   esac
+done
+# This only works if the arguments don't contain spaces; they don't.
+exec ./pngstest --tmpfile "${gamma}-${alpha}-" --log ${1+"$@"} $args

Engine/lib/lpng/tests/pngstest-1.8

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+#!/bin/sh
+exec "${srcdir}/tests/pngstest" 1.8 none

Engine/lib/lpng/tests/pngstest-1.8-alpha

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+#!/bin/sh
+exec "${srcdir}/tests/pngstest" 1.8 alpha

Engine/lib/lpng/tests/pngstest-linear

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+#!/bin/sh
+exec "${srcdir}/tests/pngstest" linear none

Engine/lib/lpng/tests/pngstest-linear-alpha

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+#!/bin/sh
+exec "${srcdir}/tests/pngstest" linear alpha

Engine/lib/lpng/tests/pngstest-none

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+#!/bin/sh
+exec "${srcdir}/tests/pngstest" none none

Engine/lib/lpng/tests/pngstest-none-alpha

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+#!/bin/sh
+exec "${srcdir}/tests/pngstest" none alpha

Engine/lib/lpng/tests/pngstest-sRGB

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+#!/bin/sh
+exec "${srcdir}/tests/pngstest" sRGB none

Engine/lib/lpng/tests/pngstest-sRGB-alpha

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+#!/bin/sh
+exec "${srcdir}/tests/pngstest" sRGB alpha

Engine/lib/lpng/tests/pngtest

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+#!/bin/sh
+exec ./pngtest --strict ${srcdir}/pngtest.png

Engine/lib/lpng/tests/pngtest-badpngs

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+#!/bin/sh
+
+# various crashers
+# using --relaxed because some come from fuzzers that don't maintain CRC's
+
+./pngtest --relaxed ${srcdir}/contrib/testpngs/crashers/badcrc.png
+./pngtest --relaxed ${srcdir}/contrib/testpngs/crashers/badadler.png
+./pngtest --xfail ${srcdir}/contrib/testpngs/crashers/bad_iCCP.png
+./pngtest --xfail ${srcdir}/contrib/testpngs/crashers/empty_ancillary_chunks.png
+./pngtest --xfail ${srcdir}/contrib/testpngs/crashers/huge_*_chunk.png \
+    ${srcdir}/contrib/testpngs/crashers/huge_*safe_to_copy.png
+
+exec ./pngtest --xfail ${srcdir}/contrib/testpngs/crashers/huge_IDAT.png

Engine/lib/lpng/tests/pngunknown-IDAT

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+#!/bin/sh
+exec ./pngunknown --strict default=discard IDAT=save "${srcdir}/pngtest.png"

Engine/lib/lpng/tests/pngunknown-discard

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+#!/bin/sh
+exec ./pngunknown --strict default=discard "${srcdir}/pngtest.png"

Engine/lib/lpng/tests/pngunknown-if-safe

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+#!/bin/sh
+exec ./pngunknown --strict default=if-safe "${srcdir}/pngtest.png"

Engine/lib/lpng/tests/pngunknown-sAPI

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+#!/bin/sh
+exec ./pngunknown --strict bKGD=save cHRM=save gAMA=save all=discard iCCP=save sBIT=save sRGB=save eXIf=save "${srcdir}/pngtest.png"

Engine/lib/lpng/tests/pngunknown-sTER

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+#!/bin/sh
+exec ./pngunknown --strict sTER=if-safe "${srcdir}/pngtest.png"

Engine/lib/lpng/tests/pngunknown-save

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+#!/bin/sh
+exec ./pngunknown --strict default=save "${srcdir}/pngtest.png"

Engine/lib/lpng/tests/pngunknown-vpAg

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+#!/bin/sh
+exec ./pngunknown --strict vpAg=if-safe "${srcdir}/pngtest.png"

Engine/lib/lpng/tests/pngvalid-gamma-16-to-8

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+#!/bin/sh
+exec ./pngvalid --strict --gamma-16-to-8

Engine/lib/lpng/tests/pngvalid-gamma-alpha-mode

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+#!/bin/sh
+exec ./pngvalid --strict --gamma-alpha-mode

Engine/lib/lpng/tests/pngvalid-gamma-background

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+#!/bin/sh
+exec ./pngvalid --strict --gamma-background

Engine/lib/lpng/tests/pngvalid-gamma-expand16-alpha-mode

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+#!/bin/sh
+exec ./pngvalid --strict --gamma-alpha-mode --expand16

Engine/lib/lpng/tests/pngvalid-gamma-expand16-background

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+#!/bin/sh
+exec ./pngvalid --strict --gamma-background --expand16

Engine/lib/lpng/tests/pngvalid-gamma-expand16-transform

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+#!/bin/sh
+exec ./pngvalid --strict --gamma-transform --expand16

Engine/lib/lpng/tests/pngvalid-gamma-sbit

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+#!/bin/sh
+exec ./pngvalid --strict --gamma-sbit

Engine/lib/lpng/tests/pngvalid-gamma-threshold

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+#!/bin/sh
+exec ./pngvalid --strict --gamma-threshold

Engine/lib/lpng/tests/pngvalid-gamma-transform

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+#!/bin/sh
+exec ./pngvalid --strict --gamma-transform

Engine/lib/lpng/tests/pngvalid-progressive-interlace-standard

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+#!/bin/sh
+exec ./pngvalid --strict --standard --progressive-read --interlace

Engine/lib/lpng/tests/pngvalid-progressive-size

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+#!/bin/sh
+exec ./pngvalid --strict --size --progressive-read

Engine/lib/lpng/tests/pngvalid-progressive-standard

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+#!/bin/sh
+exec ./pngvalid --strict --standard --progressive-read

Engine/lib/lpng/tests/pngvalid-standard

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+#!/bin/sh
+exec ./pngvalid --strict --standard

Engine/lib/lpng/tests/pngvalid-transform

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+#!/bin/sh
+exec ./pngvalid --strict --transform