Make readback of alpha channel work for RGBA.

src/gpu/gl/GrGLGpu.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLGpu_DEFINED
 #define GrGLGpu_DEFINED
 
@@ skipping 182 matching lines @@
     bool flushGLState(const DrawArgs&);
 
     // Sets up vertex attribute pointers and strides. On return indexOffsetInBytes gives the offset
     // an into the index buffer. It does not account for vertices.startIndex() but rather the start
     // index is relative to the returned offset.
     void setupGeometry(const GrPrimitiveProcessor&,
                        const GrNonInstancedVertices& vertices,
                        size_t* indexOffsetInBytes);
 
     // Subclasses should call this to flush the blend state.
-    void flushBlend(const GrXferProcessor::BlendInfo& blendInfo);
+    void flushBlend(const GrXferProcessor::BlendInfo& blendInfo,
+                    const GrSwizzle& shaderOutputSwizzle);
 
     bool hasExtension(const char* ext) const { return fGLContext->hasExtension(ext); }
 
     void copySurfaceAsDraw(GrSurface* dst,
                            GrSurface* src,
                            const SkIRect& srcRect,
                            const SkIPoint& dstPoint);
     void copySurfaceAsCopyTexSubImage(GrSurface* dst,
                                       GrSurface* src,
                                       const SkIRect& srcRect,
@@ skipping 327 matching lines @@
             SkASSERT(target == GR_GL_TEXTURE_EXTERNAL);
             return 1;
         }
     }
 
     typedef GrGpu INHERITED;
     friend class GrGLPathRendering; // For accessing setTextureUnit.
 };
 
 #endif