added support for user-defined functions to GrSKSLFP

Bug: skia:
Change-Id: I1483cdf7229b7234be41d21407e2b4abf99fff76
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/239925
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
9 files changed