[graphite] Split out ComputePathAtlas into its own file.

It's getting complicated enough that it deserves some separation from
the base class.

Bug: b/305696981
Change-Id: I8002a4565c459e8587011736e49d993042a411de
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/838738
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Arman Uguray <armansito@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
7 files changed