store SkAlphaType inside SkBitmap, on road to support unpremul

src/images/SkImageDecoder_libpng.cpp

Index: src/images/SkImageDecoder_libpng.cpp
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index b6aa329104288aead5ae32f788e0f80b9aba153d..91520f2b7b1375d1e7d2352ad0eaff4bd485d7e5 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -437,7 +437,7 @@ bool SkPNGImageDecoder::onDecode(SkStream* sk_stream, SkBitmap* decodedBitmap,
         // return false, since the result will have premultiplied colors.
         return false;
     }
-    decodedBitmap->setIsOpaque(!reallyHasAlpha);
+    decodedBitmap->setAlphaType(reallyHasAlpha ? kPremul_SkAlphaType : kOpaque_SkAlphaType);

[scroggo, 2013/10/02 18:49:58]

In addition to checking for unpremultiply, this needs to check the config for
A8, as in https://codereview.chromium.org/25726004/

[reed1, 2013/10/03 08:22:54]

Not sure what the change to the code should be. If the png is A8, I presume
reallyHasAlpha will be true, in which case I'll store kPremul. Is that not
correct?

[scroggo, 2013/10/03 14:05:19]

It doesn't currently change reallyHasAlpha, but it could. My latest patch
changes reallyHasAlpha.

This code still needs to check this->getRequireUnpremultipliedColors(). If true,
the alpha type should be kUnpremul.

[reed1, 2013/10/18 15:27:06]

Done.

     return true;
 }
 
@@ -889,7 +889,7 @@ bool SkPNGImageDecoder::onDecodeSubset(SkBitmap* bm, const SkIRect& region) {
     if (0 != theTranspColor) {
         reallyHasAlpha |= substituteTranspColor(&decodedBitmap, theTranspColor);
     }
-    decodedBitmap.setIsOpaque(!reallyHasAlpha);
+    decodedBitmap.setAlphaType(reallyHasAlpha ? kPremul_SkAlphaType : kOpaque_SkAlphaType);
 
     if (swapOnly) {
         bm->swap(decodedBitmap);