Unit test of drawAtlas and fixes for bugs uncovered by it

Change-Id: I3583a5b3930aba03ae843daade50223e87c4ac6f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/294338
Commit-Queue: Nathaniel Nifong <nifong@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
3 files changed