[viewer] Context option and UI toggle for compute MSAAx16

It's now possible to trigger the MSAAx16 mode using the path renderer
strategy toggles in viewer. The PathAtlas continues to select analytic
area AA unless MSAA16 is selected via ContextOptions.

Bug: b/285420658
Change-Id: I6f5c647ff3cb33d8ca36aa6ef363f7f883e614d5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/801377
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
5 files changed