The result SkBitmap, the pixel allocator, and the alpha
preference need to be communicated from the client to
the region decoder.
BUG=skia:
Review URL: https://codereview.chromium.org/1418093006
Use SkCodecPrintf instead of SkDebugf.
Check if the conversion is possible rather than starting many decodes
that will certainly fail.
Small refactor to code that deals with subsets that fall outside
of the image.
BUG=skia:
Review URL: https://codereview.chromium.org/1395383002