Added DSL fragmentProcessor and Sample function

Change-Id: I76348d151fbf27454a6f60e4f5f7e1774731e61b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/395216
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
10 files changed