Remove use of integers for atlas indexing

Bug: skia:
Change-Id: I7c29e90de6531a35c415f0338e23c176a7293040
Reviewed-on: https://skia-review.googlesource.com/c/171233
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed