use mul_inv instead of div for tiling

Bug: skia:
Change-Id: I5f88e7923fe204faba8dc5d87454805a4d470d52
Reviewed-on: https://skia-review.googlesource.com/20688
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
5 files changed