Revamp Metal shader creation.

Try to make this a little cleaner and set up for shader precompiles.

Change-Id: Id0cf7c88e67addfdf9f79acef4484d1d2ee2d5ad
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375199
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
4 files changed