This isn't connected to anything yet and parallels what is done
for the text GrAtlasManager.
The rough pattern is that the GrContext hierarchy implements creating
the atlas but it is only exposed/made available via the flush state
(which derives from GrDrawMeshOp::Target).
TBR=bsalomon@google.com
Bug: 1108408
Change-Id: Iad7b5648877dbd20840e8d47c1d24131098ded94
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/309128
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5edf510941