SkBitmapRegionDecoder clean-up

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
7 files changed