store SkAlphaType inside SkBitmap, on road to support unpremul

src/images/SkImageRef.cpp

Index: src/images/SkImageRef.cpp
diff --git a/src/images/SkImageRef.cpp b/src/images/SkImageRef.cpp
index 9093b0515662d24dda6023d4dedf00575a1e69a2..ead835f7ab9c9c4c16f6a5e810026855973554f6 100644
--- a/src/images/SkImageRef.cpp
+++ b/src/images/SkImageRef.cpp
@@ -64,7 +64,8 @@ bool SkImageRef::getInfo(SkBitmap* bitmap) {
 bool SkImageRef::isOpaque(SkBitmap* bitmap) {
     if (bitmap && bitmap->pixelRef() == this) {
         bitmap->lockPixels();
-        bitmap->setIsOpaque(fBitmap.isOpaque());
+        // what about colortables??????
+        bitmap->setAlphaType(fBitmap.alphaType());
         bitmap->unlockPixels();
         return true;
     }