Add support for additional textures in GrDrawOpAtlas

Step two in supporting growable/shrinkable atlases.

Bug: skia:3550
Change-Id: I0cdec2a9f59cc8ced071bfeec2f8ed5a228c4b7a
Reviewed-on: https://skia-review.googlesource.com/43260
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
12 files changed