store SkAlphaType inside SkBitmap, on road to support unpremul

src/images/SkImageDecoder.cpp

Index: src/images/SkImageDecoder.cpp
diff --git a/src/images/SkImageDecoder.cpp b/src/images/SkImageDecoder.cpp
index 1544edf366bca7939bebae7faa1faaef536dec06..52a445c78277f78e360e3c2889f5a13da5da13b3 100644
--- a/src/images/SkImageDecoder.cpp
+++ b/src/images/SkImageDecoder.cpp
@@ -242,8 +242,8 @@ bool SkImageDecoder::cropBitmap(SkBitmap *dst, SkBitmap *src, int sampleSize,
     }
     // if the destination has no pixels then we must allocate them.
     if (dst->isNull()) {
-        dst->setConfig(src->getConfig(), w, h);
-        dst->setIsOpaque(src->isOpaque());
+        dst->setConfig(src->getConfig(), w, h, 0, src->isOpaque() ?
+                       kOpaque_SkAlphaType : kPremul_SkAlphaType);
 
         if (!this->allocPixelRef(dst, NULL)) {
             SkDEBUGF(("failed to allocate pixels needed to crop the bitmap"));