Create SkSLOperators, centralize functionality

Change-Id: I3593e7ab0caac2fd572346038cbc8ff63e6fe970
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/366406
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
23 files changed