Fix multiview combined with flat interpolation 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/1146809 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>