Add support for short uniform types in Metal.

In practice we didn't have any code which uses uniform shorts, but
unit testing quickly uncovered that this conversion path was never
written. (In non-Metal layouts, getUniformTypeForLayout will convert
the shorts to 32-bit ints.)

Change-Id: I31985628d7f03e1e6d1ce592d6a4b6eb4598718e
Bug: skia:13478
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/554338
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
1 file changed