Add atan_ primitive to SkRasterPipeline.

The atan approximation logic was lifted from SkVM.

Change-Id: Id2dcc369a84666d9dafed6d7365988f906e89b18
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/635500
Reviewed-by: Herb Derby <herb@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
2 files changed