store SkAlphaType inside SkBitmap, on road to support unpremul

BUG=
R=bsalomon@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/25275004

git-svn-id: http://skia.googlecode.com/svn/trunk@11877 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/samplecode/SampleDitherBitmap.cpp b/samplecode/SampleDitherBitmap.cpp
index 7f29305..9132ceb 100644
--- a/samplecode/SampleDitherBitmap.cpp
+++ b/samplecode/SampleDitherBitmap.cpp
@@ -100,7 +100,7 @@
 
     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) {