remove 'in uniform' support from GrSkSLFP, make rules more clear

Bug: skia:
Change-Id: Iaa4d33c1bfb295d87343411ba6aacc8fae68ef9c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244300
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
11 files changed