llvmpipe: add ARB_post_depth_coverage support.

This doesn't pass thie piglits because currently they are broken
for case where GL upgrades 2 samples to 4

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5767>
5 files changed