intel/fs: Allow multiple slots for position

intel/fs: Allow multiple slots for position

Change brw_compute_vue_map() to also take the number of pos slots.  If
more than one slot is used, the VARYING_SLOT_POS is treated as an
array.

When using Primitive Replication, instead of a single position, the
VUE must contain an array of positions.  Padding might be
necessary (after clip distance) to ensure rest of attributes start
aligned.

v2: Add note about array in the commit message and assert that
    pos_slots >= 1 to make clear 0 is invalid. (Jason)
    Move padding to be after the clip distance.

v3: Apply the correct offset when gathering the sources from outputs.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net> [v2]
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2313>