Allow GrDrawOpAtlas to grow as needed

Bug: skia:3550
Change-Id: Ib5312c8c06ba8549d90545658df6686c45058255
Reviewed-on: https://skia-review.googlesource.com/45841
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
14 files changed