Clear and discard stencil buffers on tilers

Bug: skia:
Change-Id: I42e99cb75567825ac7751fc0ec56e4c45132628b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/210425
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
16 files changed