Metal: Add check for invalid shader functions

Bug: skia:8243
Change-Id: Ibcf34d72d06d06cbffc411afade277b6d94991b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/214304
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
1 file changed