Fix precedence when operator-assigning to swizzles in Metal.

Previously, we were failing to parenthesize these expressions
correctly. This was discovered by a React Native user:

https://github.com/Shopify/react-native-skia/issues/1300

Bug: skia:14123
Change-Id: I772a13ff7097bdc724b56a19b73f459809985c2c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/644317
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed