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.

Templates/Empty/game/core/scripts/client/postFx/ssao.cs

@@ -190,6 +190,7 @@ singleton PostEffect( SSAOPostFx )
    
    target = "$outTex";
    targetScale = "0.5 0.5";
+   targetViewport = "PFXTargetViewport_GFXViewport";
    
    singleton PostEffect()
    {