Reland Reland "Remove use of integers for atlas indexing"

Bug: skia:
Change-Id: Ie37b3fd6c8682b2d778b1dd0bc38976e6761524e
Reviewed-on: https://skia-review.googlesource.com/c/172142
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed