make alpha read back of bgra/rgba work

src/gpu/gl/GrGLProgramDesc.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "GrGLProgramDesc.h"
 
 #include "GrProcessor.h"
 #include "GrGLGpu.h"
@@ skipping 123 matching lines @@
     }
 
     // --------DO NOT MOVE HEADER ABOVE THIS LINE--------------------------------------------------
     // Because header is a pointer into the dynamic array, we can't push any new data into the key
     // below here.
     KeyHeader* header = glDesc->atOffset<KeyHeader, kHeaderOffset>();
 
     // make sure any padding in the header is zeroed.
     memset(header, 0, kHeaderSize);
 
+    header->fSwapDstRedAndAlpha = false;
+    if (GrPixelConfigIsAlphaOnly(pipeline.getRenderTarget()->config()) &&
+        gpu->glCaps().textureRedSupport()) {
+        header->fSwapDstRedAndAlpha = true;
+    }
+
     if (pipeline.readsFragPosition()) {
         header->fFragPosKey =
                 GrGLSLFragmentShaderBuilder::KeyForFragmentPosition(pipeline.getRenderTarget());
     } else {
         header->fFragPosKey = 0;
     }
 
     if (pipeline.ignoresCoverage()) {
         header->fIgnoresCoverage = 1;
     } else {
         header->fIgnoresCoverage = 0;
     }
 
     header->fSnapVerticesToPixelCenters = pipeline.snapVerticesToPixelCenters();
     header->fColorEffectCnt = pipeline.numColorFragmentProcessors();
     header->fCoverageEffectCnt = pipeline.numCoverageFragmentProcessors();
     glDesc->finalize();
     return true;
 }