Make readback of alpha channel work for RGBA.

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 97fcce59b777313d690bc1d7d700529b67305e9f..413874a43cf4b1b6eb88a03f4e258ce85eb2b3bb 100644
--- a/src/gpu/gl/builders/GrGLProgramBuilder.cpp
+++ b/src/gpu/gl/builders/GrGLProgramBuilder.cpp
@@ -260,6 +260,19 @@ void GrGLProgramBuilder::emitAndInstallXferProc(const GrXferProcessor& xp,
     // asks for dst color, then the emit code needs to follow suit
     verify(xp);
     fFS.codeAppend("}");
+
+    GrSwizzle swizzle;
+    swizzle.setFromKey(this->desc().header().fOutputSwizzle);
+    if (swizzle != GrSwizzle::RGBA()) {
+        fFS.codeAppendf("%s = %s.%s;", fFS.getPrimaryColorOutputName(),
+                                       fFS.getPrimaryColorOutputName(),
+                                       swizzle.c_str());
+        if (xp.hasSecondaryOutput()) {
+            fFS.codeAppendf("%s = %s.%s;", fFS.getSecondaryColorOutputName(),
+                                           fFS.getSecondaryColorOutputName(),
+                                           swizzle.c_str());
+        }
+    }
 }
 
 void GrGLProgramBuilder::verify(const GrPrimitiveProcessor& gp) {