Minor XR shader cleanup

Use TRANSFORM_TEX.
Also, there is the possibility that texcoord is being transformed in frag
too often.  However, it makes sense for tex-coord manipulation in frag.
Even so, I should check to make sure transforms are happening somewhere.

Author: robbiesri

PostProcessing/Shaders/Builtins/Copy.shader

@@ -22,7 +22,7 @@ Shader "Hidden/PostProcessing/Copy"
         {
             VaryingsDefault o;
             o.vertex = float4(v.vertex.xy * 2.0 - 1.0, 0.0, 1.0);
-            o.texcoord = v.texcoord * _MainTex_ST.xy + _MainTex_ST.zw; // We need this for VR
+            o.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex); // We need this for VR
 
             #if UNITY_UV_STARTS_AT_TOP
             o.texcoord = o.texcoord * float2(1.0, -1.0) + float2(0.0, 1.0);

PostProcessing/Shaders/StdLib.hlsl

@@ -242,6 +242,10 @@ VaryingsDefault VertDefault(AttributesDefault v)
     VaryingsDefault o;
     o.vertex = float4(v.vertex.xy, 0.0, 1.0);
     o.texcoord = TransformTriangleVertexToUV(v.vertex.xy);
+    // Most frag shaders that use texcoord do appear to use
+    // UnityStereoTransformScreenSpaceTex, but I need to
+    // manually inspect each shader.
+    // Seems like they'd be better off with the transform in the vert
 
 #if UNITY_UV_STARTS_AT_TOP
     o.texcoord = o.texcoord * float2(1.0, -1.0) + float2(0.0, 1.0);
@@ -258,4 +262,6 @@ VaryingsDefault VertDefaultNoFlip(AttributesDefault v)
     return o;
 }
 
+#define TRANSFORM_TEX(tex,name) (tex.xy * name##_ST.xy + name##_ST.zw)
+
 #endif // UNITY_POSTFX_STDLIB