store SkAlphaType inside SkBitmap, on road to support unpremul

samplecode/SampleDitherBitmap.cpp

Index: samplecode/SampleDitherBitmap.cpp
diff --git a/samplecode/SampleDitherBitmap.cpp b/samplecode/SampleDitherBitmap.cpp
index df727c42033867ddbda6039dc27a0c4a1eb0ee32..10d899f1cf672ebca6da6809ed62723af07ed2b1 100644
--- a/samplecode/SampleDitherBitmap.cpp
+++ b/samplecode/SampleDitherBitmap.cpp
@@ -101,11 +101,13 @@ protected:
 
     static void setBitmapOpaque(SkBitmap* bm, bool isOpaque) {
         SkAutoLockPixels alp(*bm);  // needed for ctable
-        bm->setIsOpaque(isOpaque);
+        bm->setAlphaType(isOpaque ? kOpaque_SkAlphaType : kPremul_SkAlphaType);
+#if 0
         SkColorTable* ctable = bm->getColorTable();
         if (ctable) {
             ctable->setIsOpaque(isOpaque);
         }
+#endif
     }
 
     static void draw2(SkCanvas* canvas, const SkBitmap& bm) {