Validate that attrib offsets don't overflow buffers. During draw calls, we wouldn't add the current attrib offset to the required draw call size when checking attributes. This could lead to us producing warnings in the D3D11 runtime, and miss returning some errors. BUG=angleproject:1339 Change-Id: I03555be396df46f83d96dfb34fbcb145169625e8 Reviewed-on: https://chromium-review.googlesource.com/331807 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>