Add a stencil test for transparent tessellated strokes

Initially we restricted tessellated stroking to opaque solid colors.
This CL adds support for transparency by enabling a stencil test. The
stencil test also allows us to use mixed samples.

Bug: skia:10419
Change-Id: Ie40f3099d2b009f92ed49f7f43e5f269b1a479af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/340798
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
10 files changed