Allow casting to lower precision types in runtime effects

Bug: skia:10679
Change-Id: If464c48b7c31d0d8440d1231d1983829d54ce598
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/315281
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
11 files changed