Cosmetic changes to GrDrawOpAtlas

It looks like we won't be going in the direction of:

https://skia-review.googlesource.com/c/skia/+/35962 (Allow GrDrawOpAtlas to grow & shrink)

Pull out what is worth keeping.

TBR=bsalomon@google.com

Change-Id: Ia14c3c621e7b1d2d7f303a455a3e4d9660af5f6e
Reviewed-on: https://skia-review.googlesource.com/41362
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
7 files changed