Better encapsulate the GrDrawOpAtlas w/in the GrSmallPathAtlasMgr

This brings the GrSmallPathAtlasMgr into closer correspondence with
the GrAtlasMgr.

It also centralizes where we update the GrSmallPathShapeData's
atlas information.

Change-Id: I892be262a85b3878dbd7f71b0503208943203d46
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310476
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed