Make readback of alpha channel work for RGBA.

src/gpu/glsl/GrGLSLCaps.h

Index: src/gpu/glsl/GrGLSLCaps.h
diff --git a/src/gpu/glsl/GrGLSLCaps.h b/src/gpu/glsl/GrGLSLCaps.h
index eba9602e5ac270cd65f68d156d5ae96d632f5e8d..136bef466c0642b3cd07547b611b5bb1c4a6cf88 100755
--- a/src/gpu/glsl/GrGLSLCaps.h
+++ b/src/gpu/glsl/GrGLSLCaps.h
@@ -108,12 +108,17 @@ public:
     /**
      * Given a texture's config, this determines what swizzle must be appended to accesses to the
      * texture in generated shader code. Swizzling may be implemented in texture parameters or a
-     * sampler rather than in the shader. In this case the shader swizzle will always be "rgba".
+     * sampler rather than in the shader. In this case the returned swizzle will always be "rgba".
      */
     const GrSwizzle& configTextureSwizzle(GrPixelConfig config) const {
         return fConfigTextureSwizzle[config];
     }
 
+    /** Swizzle that should occur on the fragment shader outputs for a given config. */
+    const GrSwizzle& configOutputSwizzle(GrPixelConfig config) const {
+        return fConfigOutputSwizzle[config];
+    }
+
     GrGLSLGeneration generation() const { return fGLSLGeneration; }
 
     /**
@@ -150,6 +155,7 @@ private:
     AdvBlendEqInteraction fAdvBlendEqInteraction;
 
     GrSwizzle fConfigTextureSwizzle[kGrPixelConfigCnt];
+    GrSwizzle fConfigOutputSwizzle[kGrPixelConfigCnt];
 
     friend class GrGLCaps;  // For initialization.