make alpha read back of bgra/rgba work

src/gpu/gl/builders/GrGLProgramBuilder.cpp

Index: src/gpu/gl/builders/GrGLProgramBuilder.cpp
diff --git a/src/gpu/gl/builders/GrGLProgramBuilder.cpp b/src/gpu/gl/builders/GrGLProgramBuilder.cpp
index dc9393db0739ea04c5acd824c903d2eb8b6f7720..6e09d701b672f5017e82ca00bfee5448043c3066 100644
--- a/src/gpu/gl/builders/GrGLProgramBuilder.cpp
+++ b/src/gpu/gl/builders/GrGLProgramBuilder.cpp
@@ -260,6 +260,15 @@ void GrGLProgramBuilder::emitAndInstallXferProc(const GrXferProcessor& xp,
     // asks for dst color, then the emit code needs to follow suit
     verify(xp);
     fFS.codeAppend("}");
+
+    if (this->desc().header().fSwapDstRedAndAlpha) {
+        fFS.codeAppendf("%s = %s.agbr;", fFS.getPrimaryColorOutputName(),
+                        fFS.getPrimaryColorOutputName());
+        if (xp.hasSecondaryOutput()) {
+            fFS.codeAppendf("%s = %s.agbr;", fFS.getSecondaryColorOutputName(),
+                            fFS.getSecondaryColorOutputName());
+        }
+    }
 }
 
 void GrGLProgramBuilder::verify(const GrPrimitiveProcessor& gp) {