SkSL: Disallow fragmentProcessor variables in several places

We only want SkSL to contain fp variables at global scope, and to sample
directly from references to those global variables. Anything else will
confuse our sample-usage analysis, and often leads to asserts in the CPP
or pipeline-stage generators.

Bug: skia:10514
Change-Id: Ie1ef10821c1b2a946a92d050fea45d95569bc934
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/304599
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed