Add asserts to make sure we always have valid GrBackendFormats before creating proxies.

Change-Id: I59dd970b6fd512f2e2ee08cc821b758b950a2b53
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/220743
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
7 files changed