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

src/gpu/gl/GrGpuGL_program.cpp

Index: src/gpu/gl/GrGpuGL_program.cpp
===================================================================
--- src/gpu/gl/GrGpuGL_program.cpp	(revision 8448)
+++ src/gpu/gl/GrGpuGL_program.cpp	(working copy)
@@ -158,7 +158,7 @@
     }
 }
 
-bool GrGpuGL::flushGraphicsState(DrawType type) {
+bool GrGpuGL::flushGraphicsState(DrawType type, const GrDeviceCoordTexture* dstCopy) {
     const GrDrawState& drawState = this->getDrawState();
 
     // GrGpu::setupClipAndFlushState should have already checked this and bailed if not true.
@@ -187,6 +187,7 @@
                                srcCoeff,
                                dstCoeff,
                                this,
+                               dstCopy,
                                &desc);
 
         fCurrentProgram.reset(fProgramCache->getProgram(desc, stages));
@@ -217,7 +218,7 @@
             color = drawState.getColor();
             coverage = drawState.getCoverage();
         }
-        fCurrentProgram->setData(this, color, coverage, &fSharedGLProgramState);
+        fCurrentProgram->setData(this, color, coverage, dstCopy, &fSharedGLProgramState);
     }
     this->flushStencil(type);
     this->flushScissor();