Add SW decode of ETC1 and a GM

The GM exercises the compressed image formats using externally created resources

Note: the original image for the new flower resources can be found on Wikimedia Commons and has a "CC0 1.0 Universal Public Domain Dedication" license.

Bug: skia:9680
Change-Id: I6c5f9a12fcbbecdc3ba548dbb078bc21522073fe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/267836
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed