sksl: Negate dFdy when the Y axis is flipped

Bug: skia:
Change-Id: Icbdaa6b1ebbe00168f57ebb888c2345d4f7a5e7d
Reviewed-on: https://skia-review.googlesource.com/c/195160
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
6 files changed