This enforces an even stricter version of the rules from GLSL ES 1.0
Appendix A, Section 5. Essentially, indices (to arrays, vectors,
matrices) must be made of literals, loop indices, and expressions made
of those two.
Bug: skia:10837
Bug: skia:11096
Change-Id: I437a5ed64da58e24d5991ddbde68859f5214e98b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/354665
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
ea485e5285