The integer precision is different on DirectX 9 and 11 backends. DirectX
11 guarantees native 32-bit integers.

BUG=angle:919

Change-Id: I7619e66f8aaa1177c32b6da185fe805ce8a19b71
Reviewed-on: https://chromium-review.googlesource.com/250280Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>