Implements missing _captureBackBuffer method for GL gfx layer.

GL layer was missing _captureBackBuffer() implementation which caused screenShot() function to fail. Also caused ScreenShot::capture() to be called every frame afterward which caused significant performance issues.

Engine/source/gfx/gl/screenshotGL.h

@@ -0,0 +1,39 @@
+//-----------------------------------------------------------------------------
+// Copyright (c) 2016 GarageGames, LLC
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to
+// deal in the Software without restriction, including without limitation the
+// rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+// sell copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+// IN THE SOFTWARE.
+//-----------------------------------------------------------------------------
+#ifndef _SCREENSHOTGL_H_
+#define _SCREENSHOTGL_H_
+
+#include "gfx/screenshot.h"
+
+//**************************************************************************
+// OpenGL implementation of screenshot
+//**************************************************************************
+class ScreenShotGL : public ScreenShot
+{
+protected:
+
+   GBitmap* _captureBackBuffer();
+
+};
+
+
+#endif  // _SCREENSHOTGL_H_