Add BC1_RGB8_UNORM support

This is working in the GL and Vulkan back ends. MacOS only supports the RGBA8 variants.
For mobile devices, probably only nVidia GPUs will support this.

Bug: skia:9680
Change-Id: I9d886b72232a031603e93e46059a97a8aa288b3c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261093
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
19 files changed