Move more atlas functionality from GrSmallPathRenderer to GrSmallPathAtlasMgr

After this CL the GrSmallPathAtlasMgr is almost all ready to be moved
to be a flush-time entity.

This is pulled out of the omnibus CL:
https://skia-review.googlesource.com/c/skia/+/307776 (Split the small path renderer into record-time and flush-time pieces)

Change-Id: Ie6a2196a640ce9be7a1e1c6d7bbdfd41413c42d0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/308977
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed