Use unsigned arithmetic for SkBitmapProcStateAutoMapper bias

To avoid undefined int underflow behavior.

BUG=skia:6017
R=mtklein@google.com,reed@google.com

Change-Id: Ib707ad5e1d87eda70525c56db14849c4164c5640
Reviewed-on: https://skia-review.googlesource.com/5861
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed