make alpha read back of bgra/rgba work

src/gpu/gl/GrGLCaps.h

Index: src/gpu/gl/GrGLCaps.h
diff --git a/src/gpu/gl/GrGLCaps.h b/src/gpu/gl/GrGLCaps.h
index 026ae23fb7af08271043c1fbcb4e4ac2c081178c..c92a973d7130f946a8f06cfa88565fea4cc4903b 100644
--- a/src/gpu/gl/GrGLCaps.h
+++ b/src/gpu/gl/GrGLCaps.h
@@ -235,7 +235,7 @@ public:
     bool readPixelsSupported(const GrGLInterface* intf,
                              GrGLenum format,
                              GrGLenum type,
-                             GrGLenum currFboFormat) const;
+                             GrPixelConfig currRTConfig) const;
 
     bool isCoreProfile() const { return fIsCoreProfile; }
 
@@ -378,14 +378,22 @@ private:
     bool fExternalTextureSupport : 1;
 
     struct ReadPixelsSupportedFormat {
-        GrGLenum fFormat;
-        GrGLenum fType;
-        GrGLenum fFboFormat;
+        ReadPixelsSupportedFormat() { memset(this, 0, sizeof(ReadPixelsSupportedFormat)); }
+        ReadPixelsSupportedFormat(GrGLenum format, GrGLenum type, GrPixelConfig rtConfig) {
+            memset(this, 0, sizeof(ReadPixelsSupportedFormat));
+            fFormat = format;
+            fType = type;
+            fRTConfig = rtConfig;
+        }
+
+        GrGLenum        fFormat;
+        GrGLenum        fType;
+        GrPixelConfig   fRTConfig;
 
         bool operator==(const ReadPixelsSupportedFormat& rhs) const {
             return fFormat    == rhs.fFormat
                 && fType      == rhs.fType
-                && fFboFormat == rhs.fFboFormat;
+                && fRTConfig  == rhs.fRTConfig;
         }
     };
     mutable SkTHashMap<ReadPixelsSupportedFormat, bool> fReadPixelsSupportedCache;