Make readback of alpha channel work for RGBA.

src/gpu/gl/builders/GrGLProgramBuilder.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrGLProgramBuilder.h"
 
 #include "GrAutoLocaleSetter.h"
@@ skipping 242 matching lines @@
                                        ignoresCoverage ? nullptr : coverageIn.c_str(),
                                        fFS.getPrimaryColorOutputName(),
                                        fFS.getSecondaryColorOutputName(),
                                        samplers);
     fXferProcessor->fGLProc->emitCode(args);
 
     // We have to check that effects and the code they emit are consistent, ie if an effect
     // asks for dst color, then the emit code needs to follow suit
     verify(xp);
     fFS.codeAppend("}");
+
+    GrSwizzle swizzle;
+    swizzle.setFromKey(this->desc().header().fOutputSwizzle);
+    if (swizzle != GrSwizzle::RGBA()) {
+        fFS.codeAppendf("%s = %s.%s;", fFS.getPrimaryColorOutputName(),
+                                       fFS.getPrimaryColorOutputName(),
+                                       swizzle.c_str());
+        if (xp.hasSecondaryOutput()) {
+            fFS.codeAppendf("%s = %s.%s;", fFS.getSecondaryColorOutputName(),
+                                           fFS.getSecondaryColorOutputName(),
+                                           swizzle.c_str());
+        }
+    }
 }
 
 void GrGLProgramBuilder::verify(const GrPrimitiveProcessor& gp) {
     SkASSERT(fFS.hasReadFragmentPosition() == gp.willReadFragmentPosition());
 }
 
 void GrGLProgramBuilder::verify(const GrXferProcessor& xp) {
     SkASSERT(fFS.hasReadDstColor() == xp.willReadDstColor());
 }
 
@@ skipping 205 matching lines @@
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 GrGLInstalledFragProcs::~GrGLInstalledFragProcs() {
     int numProcs = fProcs.count();
     for (int i = 0; i < numProcs; ++i) {
         delete fProcs[i];
     }
 }