This adds the same tessellator code that swr uses, swr should
move to using this copy, unfortunately that wasn't trivial on my first
look.
The p_tessellator wrapper wraps it in a form that is a useful interface
for draw.
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3841>
Dave Airlie