Added support in ANGLE for the provoking vertex feature,
which allows to choose the provoking vertex convention
(first or last) for flat shading.

Just copying the vk_ext_provoking_vertex.h file into ANGLE src tree for now.
When the extension lands in the vulkan header we'll migrate to that and remove this
file from ANGLE.

With this, all these tests pass with SwANGLE:
dEQP-GLES3.functional.fragment_out.* (fixes 526 failures)
dEQP-GLES3.functional.rasterization.flatshading.* (fixes 6 failures)
dEQP-GLES3.functional.shaders.linkage.varying.* (already passing before, still passing)

Bug: angleproject:3430
Bug: angleproject:3677
Bug: angleproject:4063
Change-Id: Icc361f567072c12472398e37a94a61d7f95007ad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1855681
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Alexis Hétu <sugoi@chromium.org>