Set the GS layer provoking vertex value to
GL_LAST_VERTEX_CONVENTION if the provoking vertex extension
is enabled. Otherwise, use GL_FIRST_VERTEX_CONVENTION as
vulkan follows this convention for provoking vertex.

Tests: dEQP-GLES31.functional.geometry_shading.layered.layer_provoking_vertex_*
Bug: angleproject:5452
Change-Id: Ie385f3b713486c54114dbfcefd799d180701bb60
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2799033
Commit-Queue: Brandon Schade <b.schade@samsung.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>