fixed some issues with Metal shader functions

The recent change to output child processors as functions served as a
stress test for Metal functions and uncovered some issues with
requirements tracking. This should get everything back on track.

Bug: skia:
Change-Id: If683c95e6668f1afd7be24195071a26cdb84d660
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/230130
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
3 files changed