store SkAlphaType inside SkBitmap, on road to support unpremul

samplecode/SampleTinyBitmap.cpp

Index: samplecode/SampleTinyBitmap.cpp
diff --git a/samplecode/SampleTinyBitmap.cpp b/samplecode/SampleTinyBitmap.cpp
index 16cbce4adc7e038c23116f3ef3974734b00bd49e..27c55214e50559e98409bf1d405fb281ff7f03fd 100644
--- a/samplecode/SampleTinyBitmap.cpp
+++ b/samplecode/SampleTinyBitmap.cpp
@@ -57,11 +57,13 @@ 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
     }
 
     virtual void onDrawContent(SkCanvas* canvas) {