On D3D the vertex ID is always indexed from 0, unlike GL. The D3D
backend had assumed the wrong behavior. This forwards the true offset to
D3D by using the ConstantsBuffer.

Bug: angleproject:3090
Change-Id: Ia19e3490503c97541af14979b75af0c94c67ab6b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1520988
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>