Added FXAA post-processing

Implementation of Fast Approximate Anti-Aliasing is based on nVidia sample:
https://github.com/NVIDIAGameWorks/GraphicsSamples/tree/master/samples/es3-kepler/FXAA

src/gl/scene/gl_scene.cpp

@@ -832,6 +832,7 @@ sector_t * FGLRenderer::RenderViewpoint (AActor * camera, GL_IRECT * bounds, flo
 			TonemapScene();
 			ColormapScene();
 			LensDistortScene();
+			ApplyFXAA();
 
 			// This should be done after postprocessing, not before.
 			mBuffers->BindCurrentFB();