Add support for reading the dst pixel value in an effect. Use in a new effect for the kDarken xfer …

src/gpu/gl/GrGLProgram.h

Index: src/gpu/gl/GrGLProgram.h
===================================================================
--- src/gpu/gl/GrGLProgram.h	(revision 8448)
+++ src/gpu/gl/GrGLProgram.h	(working copy)
@@ -102,7 +102,11 @@
      *
      * The color and coverage params override the GrDrawState's getColor() and getCoverage() values.
      */
-    void setData(GrGpuGL*, GrColor color, GrColor coverage, SharedGLState*);
+    void setData(GrGpuGL*,
+                 GrColor color,
+                 GrColor coverage,
+                 const GrDeviceCoordTexture* dstCopy, // can be NULL
+                 SharedGLState*);
 
 private:
     GrGLProgram(const GrGLContext& gl,
@@ -154,11 +158,18 @@
         UniformHandle       fColorUni;
         UniformHandle       fCoverageUni;
         UniformHandle       fColorFilterUni;
+
         // We use the render target height to provide a y-down frag coord when specifying
         // origin_upper_left is not supported.
         UniformHandle       fRTHeightUni;
+
+        // Uniforms for computing texture coords to do the dst-copy lookup
+        UniformHandle       fDstCopyTopLeftUni;
+        UniformHandle       fDstCopyScaleUni;
+        UniformHandle       fDstCopySamplerUni;
+
         // An array of sampler uniform handles for each effect.
-        SamplerUniSArray    fSamplerUnis[GrDrawState::kNumStages];
+        SamplerUniSArray    fEffectSamplerUnis[GrDrawState::kNumStages];
 
         UniformHandles() {
             fViewMatrixUni = GrGLUniformManager::kInvalidUniformHandle;
@@ -166,6 +177,9 @@
             fCoverageUni = GrGLUniformManager::kInvalidUniformHandle;
             fColorFilterUni = GrGLUniformManager::kInvalidUniformHandle;
             fRTHeightUni = GrGLUniformManager::kInvalidUniformHandle;
+            fDstCopyTopLeftUni = GrGLUniformManager::kInvalidUniformHandle;
+            fDstCopyScaleUni = GrGLUniformManager::kInvalidUniformHandle;
+            fDstCopySamplerUni = GrGLUniformManager::kInvalidUniformHandle;
         }
     };