zink: implement Vk_EXT_index_type_uint8

this is a simple extension that enables using uint8-sized index buffers,
which lets us avoid having those go through primconvert

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5712>
3 files changed