Add compression to Mock backend

Bring the Mock backend up to speed since we're expanding compression support.

Bug: skia:9680
Change-Id: I7fd7bed9351d3534d238feca0add54eced31cb4b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/259099
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
14 files changed