The geometry shader we want will depend on our current draw mode, and
if we're using flat shading in the shader. Without flat shading, we'll
still be using them only for point sprites, but for other primitive
types with flat shading enabled, we'll be using them to correct the
provoking vertex order with D3D11.

Note: no new features in this CL, those are turned on in follow-ups.

BUG=angleproject:754

Change-Id: Iabf13ffd582f5a7200ee0df5aa9c3671aa7b6ed4
Reviewed-on: https://chromium-review.googlesource.com/309154Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>