Require stencil bits on wrapped GrBackendRenderTargets to 0, 8, or 16.

This will allow us in a follow on change to put in a "fake" gl stencil
format for the wrapped FBO.

Bug: skia:10727
Change-Id: Iba1eca5114b34d25e6a220591d28a20f7b36b12b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/321545
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
3 files changed