Fix the calculation of the D3D stream output offset in the D3D11 backend Also add an extra check in BindBufferBase/Range that XFB is not active. BUG=angle:898 Change-Id: I0d61ebaa0674b0a375cc35efa614b021a6901eac Reviewed-on: https://chromium-review.googlesource.com/243892 Tested-by: Gregoire Payen de La Garanderie <Gregory.Payen@imgtec.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>