Support the Boolean form of mix() in the Metal/SPIR-V backends.

Unfortunately, it's not easily testable since SkSLTest doesn't support
non-ES2 intrinsics.

Change-Id: I5b11e2af55fdc6fc29f48725aa977471c738b55e
Bug: skia:11222
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/426538
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed