added external value support to SkSL interpreter

Bug: skia:
Change-Id: I1806e1f6abaef322ecd3ba0d9bfaa9d7ca7a44c1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212983
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
18 files changed