Add a mustWriteToFragColor() workaround flag

Bug: skia:
Change-Id: Ifeff01125ebe5418d4632ffe2422140944e44469
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206351
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
4 files changed