Added grayscale post processing shader

git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@3439 8ced0084-cf51-0410-be5f-012b33b47a6e

Data/User/Shaders/grayscale.txt

@@ -0,0 +1,8 @@
+uniform samplerRECT samp0 : register(s0);
+
+void main(out float4 ocol0 : COLOR0, in float2 uv0 : TEXCOORD0)
+{
+  float4 c0 = texRECT(samp0, uv0).rgba;
+  float avg = (c0.r + c0.g + c0.b) / 3.0;
+  ocol0 = float4(avg, avg, avg, c0.a);
+}