Fix old use of GrBackendRenderTarget for GL

Change-Id: I010b9de920d5eddae7556a825ea85a208f47e06f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/744077
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
1 file changed