Viewport fix for PostFX and SSAO

- General fix for PostFX render targets to properly support the GFX
viewport setting.  This is an opt-in change through the use of the new
mTargetViewport property as most PostFX run fine with their assumption
of the viewport being the whole rendering target.
- The SSAO PostFX has been modified to use the new mTargetViewport
property.  This allows correct rendering in a side-by-side view such as
with the Oculus Rift, or any other constrained viewport rendering.

Engine/source/postFx/postEffectCommon.h

@@ -67,6 +67,20 @@ enum PFXTargetClear
 
 DefineEnumType( PFXTargetClear );
 
+
+/// PFXTargetViewport specifies how the viewport should be
+/// set up for a PostEffect's target.
+enum PFXTargetViewport
+{
+   /// The default viewport set up to match the target size
+   PFXTargetViewport_TargetSize,
+
+   /// Use the current GFX viewport
+   PFXTargetViewport_GFXViewport,
+};
+
+DefineEnumType( PFXTargetViewport );
+
 ///
 struct PFXFrameState
 {