The enables geometry shaders that correct for the flat shading on
provoking vertexes. It does not fix it for triangle strips, or in
conjunction with primitive restart (which is not yet implemented
in D3D11).

Also ensure we do not regress with flat shading enabled and transform
feedback. In cases where we use flat shading, do a double draw call,
first with an untransformed vertex stream, and no geometry shader,
then with the geometry shader enabled.

This also fixes the dEQP fragment output tests with ints.

BUG=angleproject:754

Change-Id: Ib37e8ec32d19db17ea5d4dc88158cdae0c956bfc
Reviewed-on: https://chromium-review.googlesource.com/309155
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>