Add support for atan() to DSL.

Change-Id: I00cc1e89fd85fdc0ce0860fcb35ececd0eaec50a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/400540
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
6 files changed