[canvaskit] Request an 8 bit stencil buffer from the WebGL 1/2 context

We hadn't been requesting one, but told Skia we had one, which caused
issues.

Bug: skia:9564
Change-Id: I9a2e78a528778a386225cdcbbb2d2b3d8f705f05
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254801
Reviewed-by: Brian Salomon <bsalomon@google.com>
2 files changed