Fix SW decompression of BC1 and add a GM to check it.

Bug: skia:9680
Change-Id: I8a8f9ee1aad783fc247c5516ac1effd0dab25936
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268158
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed