added SkSL DSL uniform support

Change-Id: I42c9aa76318ed744ba1390733bb0bd33413ebf29
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/362499
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
8 files changed