add optional SkAlphaType parameter to notifyPixelsChanged

src/core/SkBitmap.cpp

Index: src/core/SkBitmap.cpp
diff --git a/src/core/SkBitmap.cpp b/src/core/SkBitmap.cpp
index 9227906eab643a60d51e008d0a1d631176c81419..cfe646c595e7f648ded0ec06c04f0c93e06dab27 100644
--- a/src/core/SkBitmap.cpp
+++ b/src/core/SkBitmap.cpp
@@ -322,6 +322,9 @@ bool SkBitmap::setAlphaType(SkAlphaType alphaType) {
         return false;
     }
     fAlphaType = SkToU8(alphaType);
+    if (fPixelRef) {
+        fPixelRef->notifyAlphaTypeChanged(alphaType);

[scroggo, 2014/01/24 20:50:52]

One downside is that if a bitmap representing an opaque subset calls this, the
full set has its alpha type changed. I think we all agreed we don't care about
such a use case, so that's fine.

Maybe we could put a comment in the doc that it will affect all bitmaps pointing
to the same pixel ref (which I assume Hal won't like because it means more
exposure for pixel refs). I'm fine with or without the added comment.

[reed1, 2014/01/27 13:46:14]

Done.

+    }
     return true;
 }