make alpha read back of bgra/rgba work

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 e3d292c062aedefdee962693097377a865e3598f..e032d514f3b37b32fa22059e4cb3dc64e77bca53 100644
--- a/src/gpu/gl/GrGLProgramDesc.cpp
+++ b/src/gpu/gl/GrGLProgramDesc.cpp
@@ -141,6 +141,12 @@ bool GrGLProgramDescBuilder::Build(GrProgramDesc* desc,
     // 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());