make alpha read back of bgra/rgba work

src/gpu/gl/GrGLCaps.cpp

Index: src/gpu/gl/GrGLCaps.cpp
diff --git a/src/gpu/gl/GrGLCaps.cpp b/src/gpu/gl/GrGLCaps.cpp
index a7e957cf4d1d1fb60c4efe0feb7515ba5d835555..3adde0641501a7c2d5bbcfbcecb9c8a4f3283ed7 100644
--- a/src/gpu/gl/GrGLCaps.cpp
+++ b/src/gpu/gl/GrGLCaps.cpp
@@ -997,8 +997,8 @@ bool GrGLCaps::doReadPixelsSupported(const GrGLInterface* intf,
 bool GrGLCaps::readPixelsSupported(const GrGLInterface* intf,
                                    GrGLenum format,
                                    GrGLenum type,
-                                   GrGLenum currFboFormat) const {
-    ReadPixelsSupportedFormat key = {format, type, currFboFormat};
+                                   GrPixelConfig currRTConfig) const {
+    ReadPixelsSupportedFormat key = {format, type, currRTConfig};
     if (const bool* supported = fReadPixelsSupportedCache.find(key)) {
         return *supported;
     }