Fix OOB access for dynamic attribs with offsets. 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>