llvmpipe: multisample sample mask + early/late depth pass

llvmpipe: multisample sample mask + early/late depth pass

Start adding support for multisample masks and the depth passes

The depth passes have to run per-sample, this isn't complete support
it adds the loops, and handles the execution masks.

One mask is stored per sample, they are combined post the early Z
pass into a single shader execution mask, and then the resulting
shader execution mask is anded back in for the late Z pass.

Init the vars to NULL to avoid gcc warnings

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4122>