Fix SW decompression of BC1 and add a GM to check it (take 2).

Bug: skia:9680
Change-Id: I59a4c18f7ba7d04162715cfcbeefd70f84609edb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268439
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed