nir: Add a lowering pass for point smoothing

When point smoothing is enabled then this lowering pass will
modifies the alpha component of every write to fragment output.
Anti-aliased points get rounded with respect to their radius instead
of square.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15117>

src/compiler/nir/meson.build

@@ -187,6 +187,7 @@ files_libnir = files(
   'nir_lower_pntc_ytransform.c',
   'nir_lower_point_size.c',
   'nir_lower_point_size_mov.c',
+  'nir_lower_point_smooth.c',
   'nir_lower_poly_line_smooth.c',
   'nir_lower_printf.c',
   'nir_lower_regs_to_ssa.c',