Add alph8 gainmap decoding with BigmapRegionDecoder.

Bug: b/271174821
Change-Id: I854ace468bb385dcd4391f7bb7801caa65563e87
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/696480
Commit-Queue: Nolan Scobie <nscobie@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed