Have GrGlyphCache and GrAtlasManager compute the atlas limits independently

DDL contexts will have a GrGlyphCache but not a GrAtlasManager. This CL
disentangles the computation of the atlas limits so the GrGlyphCache
can function independently.

Change-Id: Ia698c7d1ec625d1a0d1f0b5521b56731cfeafde9
Reviewed-on: https://skia-review.googlesource.com/112708
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
5 files changed