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>

src/intel/compiler/brw_fs_visitor.cpp

@@ -698,8 +698,18 @@ fs_visitor::emit_urb_writes(const fs_reg &gs_vertex_count)
                sources[length++] = reg;
             }
          } else {
-            for (unsigned i = 0; i < 4; i++)
-               sources[length++] = offset(this->outputs[varying], bld, i);
+            int slot_offset = 0;
+
+            /* When using Primitive Replication, there may be multiple slots
+             * assigned to POS.
+             */
+            if (varying == VARYING_SLOT_POS)
+               slot_offset = slot - vue_map->varying_to_slot[VARYING_SLOT_POS];
+
+            for (unsigned i = 0; i < 4; i++) {
+               sources[length++] = offset(this->outputs[varying], bld,
+                                          i + (slot_offset * 4));
+            }
          }
          break;
       }