This getter will now return the offset to bind along with the
ID3D11Buffer pointer. This encapsulates the logic of the emulation
so that we only actually check the capabilities of the driver in
a single place. This solves the issue of needing to edit the code
in multiple places when trying to force the caps for debugging,
and also encapsulates the offset calculation in the Buffer11 class.

BUG=chromium:593024

Change-Id: Idb09d560868ae12b98bcaf4ea031c4e0b6c82b4a
Reviewed-on: https://chromium-review.googlesource.com/411918Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>