We can no longer rely on the `patch_control_point<>` array being
present, so the best we can do is use the value given us at compile
time.
This was an oversight on my part when I initially implemented the
raw-buffer tessellation evaluation input mode. The lack of tests for the
`PatchVertices` built-in almost certainly contributed, so I fixed that
in this patch.
Fixes the test
`dEQP-VK.tessellation.shader_input_output.patch_vertices_in_tes`. This
is the last failing test under `dEQP-VK.tessellation`.
