Extract a GrDynamicAtlas base class from GrCCAtlas

Change-Id: I85fd3a3ccc34b5616a75e2c7ddcc33af18809144
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268723
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
9 files changed