Support creation/use of mipped proxy in GrBackendTextureImageGenerator

Bug: skia:
Change-Id: I9d06780ccb2db0865100b67041c03408f2445c62
Reviewed-on: https://skia-review.googlesource.com/61241
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
4 files changed