sksl: Polyfill fma() when not present in GLSL

Bug: skia:
Change-Id: Ibaf57ac2feb786ed45b42e5cb23d231a2204fb23
Reviewed-on: https://skia-review.googlesource.com/c/171420
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
5 files changed