Add SkSL DSLVar

Change-Id: I1093aa6bbdb481c98ea3dab10c06bfcf323b2a75
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/352058
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
11 files changed