Make readback of alpha channel work for RGBA.

src/gpu/gl/GrGLProgramDesc.cpp

Index: src/gpu/gl/GrGLProgramDesc.cpp
diff --git a/src/gpu/gl/GrGLProgramDesc.cpp b/src/gpu/gl/GrGLProgramDesc.cpp
index 032562550cec5d5541f5405f77fcb861e8b046ed..31eeffac4e9566af1785592d81a8d4dd2721ef37 100644
--- a/src/gpu/gl/GrGLProgramDesc.cpp
+++ b/src/gpu/gl/GrGLProgramDesc.cpp
@@ -42,7 +42,7 @@ static void add_texture_key(GrProcessorKeyBuilder* b, const GrProcessor& proc,
  * function because it is hairy, though FPs do not have attribs, and GPs do not have transforms
  */
 static bool gen_meta_key(const GrProcessor& proc,
-                         const GrGLCaps& caps,
+                         const GrGLSLCaps& glslCaps,
                          uint32_t transformKey,
                          GrProcessorKeyBuilder* b) {
     size_t processorKeySize = b->size();
@@ -54,7 +54,7 @@ static bool gen_meta_key(const GrProcessor& proc,
         return false;
     }
 
-    add_texture_key(b, proc, *caps.glslCaps());
+    add_texture_key(b, proc, glslCaps);
 
     uint32_t* key = b->add32n(2);
     key[0] = (classID << 16) | SkToU32(processorKeySize);
@@ -75,7 +75,7 @@ static bool gen_frag_proc_and_meta_keys(const GrPrimitiveProcessor& primProc,
     fp.getGLSLProcessorKey(*caps.glslCaps(), b);
 
     //**** use glslCaps here?
-    return gen_meta_key(fp, caps, primProc.getTransformKey(fp.coordTransforms(),
+    return gen_meta_key(fp, *caps.glslCaps(), primProc.getTransformKey(fp.coordTransforms(),
                                                            fp.numTransformsExclChildren()), b);
 }
 
@@ -83,6 +83,8 @@ bool GrGLProgramDescBuilder::Build(GrProgramDesc* desc,
                                    const GrPrimitiveProcessor& primProc,
                                    const GrPipeline& pipeline,
                                    const GrGLGpu* gpu) {
+    const GrGLSLCaps& glslCaps = *gpu->glCaps().glslCaps();
+
     // The descriptor is used as a cache key. Thus when a field of the
     // descriptor will not affect program generation (because of the attribute
     // bindings in use or other descriptor field settings) it should be set
@@ -97,9 +99,8 @@ bool GrGLProgramDescBuilder::Build(GrProgramDesc* desc,
 
     GrProcessorKeyBuilder b(&glDesc->key());
 
-    primProc.getGLSLProcessorKey(*gpu->glCaps().glslCaps(), &b);
-    //**** use glslCaps here?
-    if (!gen_meta_key(primProc, gpu->glCaps(), 0, &b)) {
+    primProc.getGLSLProcessorKey(glslCaps, &b);
+    if (!gen_meta_key(primProc, glslCaps, 0, &b)) {
         glDesc->key().reset();
         return false;
     }
@@ -115,7 +116,7 @@ bool GrGLProgramDescBuilder::Build(GrProgramDesc* desc,
     const GrXferProcessor& xp = pipeline.getXferProcessor();
     xp.getGLSLProcessorKey(*gpu->glCaps().glslCaps(), &b);
     //**** use glslCaps here?
-    if (!gen_meta_key(xp, gpu->glCaps(), 0, &b)) {
+    if (!gen_meta_key(xp, glslCaps, 0, &b)) {
         glDesc->key().reset();
         return false;
     }
@@ -128,6 +129,9 @@ bool GrGLProgramDescBuilder::Build(GrProgramDesc* desc,
     // make sure any padding in the header is zeroed.
     memset(header, 0, kHeaderSize);
 
+    header->fOutputSwizzle =
+        glslCaps.configOutputSwizzle(pipeline.getRenderTarget()->config()).asKey();
+
     if (pipeline.readsFragPosition()) {
         header->fFragPosKey =
                 GrGLSLFragmentShaderBuilder::KeyForFragmentPosition(pipeline.getRenderTarget());