Add gl_SampleMask functionality to fragment builders

Adds methods for overriding and masking a fragment's sample mask.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1690963003

Committed: https://skia.googlesource.com/skia/+/533cefe5b9c7cec2592fc7ca00ee4cf69a26c094

Review URL: https://codereview.chromium.org/1690963003
10 files changed