Add extra parentheses as required by WGSL spec.

Context: https://www.w3.org/TR/WGSL/#operator-precedence-associativity
This resolves `error: mixing _ and _ requires parentheses` in a few
spots. It adds more parens than are strictly necessary, but not
so many that the code becomes a mess.

Change-Id: I32d8c5a604f5d0cb8db507f67e9c36fdcb49891c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/717337
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
145 files changed