Add SkImage::CompressionType::kBC1_RGBA8_UNORM support

This is mainly so we can test the compression code on macOS.

Bug: skia:9680
Change-Id: Ie0a2eacfe9100ee4ce4cc94c878d3032d6985832
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/264480
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
23 files changed