support for 'half' types in sksl, plus general numeric type improvements

Bug: skia:
Change-Id: Id285262fda8291847f11343d499b5df62ddb4b09
Reviewed-on: https://skia-review.googlesource.com/28980
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
16 files changed