Improve handling of GrPixelConfig in GrBackendTex/RT ctors

Make sure that no client facing code was relying on what we set as the
default value for fConfig by making in kUnkown.

Bug: skia:
Change-Id: Ie52ff08ba8deeacc16fe06eb0dd0c7292b2edf91
Reviewed-on: https://skia-review.googlesource.com/114261
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
17 files changed