change setAlphaType to not modify the pixelref's genID

src/core/SkPixelRef.cpp

Index: src/core/SkPixelRef.cpp
diff --git a/src/core/SkPixelRef.cpp b/src/core/SkPixelRef.cpp
index 507a4fcd48c92a0849129aa85c5aabef2bfedacb..d08796b18c818aaca263b128de5091f63d3a67bd 100644
--- a/src/core/SkPixelRef.cpp
+++ b/src/core/SkPixelRef.cpp
@@ -254,17 +254,20 @@ void SkPixelRef::callGenIDChangeListeners() {
     fGenIDChangeListeners.deleteAll();
 }
 
-void SkPixelRef::notifyPixelsChanged(SkAlphaType at) {
+void SkPixelRef::notifyPixelsChanged() {
 #ifdef SK_DEBUG
     if (fIsImmutable) {
         SkDebugf("========== notifyPixelsChanged called on immutable pixelref");
     }
 #endif
-    *const_cast<SkAlphaType*>(&fInfo.fAlphaType) = at;
     this->callGenIDChangeListeners();
     this->needsNewGenID();
 }
 
+void SkPixelRef::changeAlphaType(SkAlphaType at) {
+    *const_cast<SkAlphaType*>(&fInfo.fAlphaType) = at;
+}
+
 void SkPixelRef::setImmutable() {
     fIsImmutable = true;
 }