create SkAtlasSubRun API subclass SkMaskSubRun

The SkMaskSubRun is just a temporary name while I pull out the
different subclasses.

Change-Id: I363742a98a0596ba3d282b918cc0492ab1e0e5d9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303357
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed