Move addToAtlas to GrAtlasManager

Change-Id: I24cee3587da3dabfc618e2c028d37bbdac52cde0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293938
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
5 files changed