Wrap 'u_rtFlip.y * dfdy()' in parentheses, in Metal.

This guards against unexpected results when dfdy is used in complex
expressions. In practice, I'm not aware of this causing any trouble.

Change-Id: I639bef465d7907049d79681a49f9be67b4c435a6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/465378
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed