Add optional param to setBackendTextureState to return previous state.

Bug: skia:10742
Change-Id: I334e7896d0a1509eb666c46d5731d2573a5c1aba
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/318698
Reviewed-by: Austin Eng <enga@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
10 files changed