We were not properly adding the offset to compute the right bounds.

Bug: chromium:943709
Change-Id: I93e714b46dd366d5833fffa858ea3ab0322ffa92
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1548441
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>