Added SkSL DSLFunction

Change-Id: Ibc995e908e5b4f8d1516e13d56854a4fcf5cc809
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/360556
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
12 files changed