Add ability to query gl format enum from GrBackendTexture.

Change-Id: I5faec55b8fddb718999dae8c6b2e7badb6a85efb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/515178
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed