[graphite] Stub in SoftwarePathAtlas.

Adds the SoftwarePathAtlas class, and the location in the snapping
process where we would upload the software mask data.

Bug: b/291735945
Change-Id: If7bb59b52b60b76c0bf49b52a16cc66698999ee7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/753498
Reviewed-by: Arman Uguray <armansito@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
6 files changed