[graphite] Progress towards caching rasterPathAtlas masks.

Adds the base data structures used for caching and multiple pages,
and hooks them up for the current single page case.

Bug: b/291735945
Change-Id: I88ea3d86dfa5af59c061e110499af8c608c4bd64
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/761863
Reviewed-by: Arman Uguray <armansito@google.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed