Previously, this would generate invalid code such as `[[user(locn-1)]]`.
We now generate a more-useful error at SkSL compilation time.
Change-Id: Ifbe335ec6d4abcbdfe89b892ba51063c94d22b11
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/339397
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
