[graphite] More steps towards a shared DrawAtlas class.

* Move Rectanizer classes to a shared location
* Have GrDrawOpAtlas store SkColorType and explicit bytes-per-pixel
  instead of GrColorType.

Bug: skia:13118
Change-Id: Ib5c3d79394c89dce7f06e8eddf09a5f6a9543a7f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/531320
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
20 files changed