Add a workaround to allow sample variables for stencil draws

gl_SampleMask seems broken in general on Adreno and Radeon, but hasn't
proven to be an issue yet when rendering only to stencil.

Bug: skia:
Change-Id: I67c6b5bc2ecc3667381bd43f7bb3849c723490ec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213051
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
3 files changed