PostProcessing: Add support for user-supplied anaglyph shaders.

There are lots of different anaglyph glasses out there and there may be even more creative uses for stereoscopic post-processing shaders.

Data/Sys/Shaders/Anaglyph/grayscale.glsl

@@ -0,0 +1,10 @@
+// Anaglyph Red-Cyan grayscale shader
+
+void main()
+{
+	float4 c0 = SampleLayer(0);
+	float avg0 = (c0.r + c0.g + c0.b) / 3.0;
+	float4 c1 = SampleLayer(1);
+	float avg1 = (c1.r + c1.g + c1.b) / 3.0;
+	SetOutput(float4(avg0, avg1, avg1, c0.a));
+}