implemented mustImplementGSInvocationsWithLoop workaround in sksl

Bug: skia:
Change-Id: I8434be89537a8baf9e9c5c7a643d28c03108f4ea
Reviewed-on: https://skia-review.googlesource.com/18154
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
8 files changed