Adjust the alpha type for pixelRefs.

Move SkBitmap's validate_alphaType to SkImageInfo, with the new
name SkColorTypeValidateAlphaType. Use it in SkPixelRef's constructors,
as well as in SkDecodingImageGenerator. This fixes a bug where an
SkPixelRef's SkAlphaType could get out of sync with its SkBitmap,
when both were assigned the same SkAlphaType.

R=reed@google.com, halcanary@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/346593003
5 files changed