Add support for half-precision types in Metal.

This will hopefully improve performance on lower-end GPUs.

Change-Id: I9c2ee6dc31acd08bec0bfb5f59edc3cf90163f9e
Bug: skia:12339
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/465078
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
253 files changed