Declaring HLSL outputs with SV_* semantics after the ones with
TEXCOORD* semantics makes multiview shaders that contain a flat
varying run correctly. This is a workaround for what seems to be a
bug in the DX runtime, reproducible on at least NVIDIA and Intel.

This also adds another, clearer test case for flat interpolation used
together with multiview.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: I127b85497aa4fee58b74893165a980cbdd4444d6
Reviewed-on: https://chromium-review.googlesource.com/1146809Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>