wsi: Metal support

Lavapipe can render to a Metal surface now.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31208>

meson_options.txt

@@ -12,7 +12,7 @@ option(
   type : 'array',
   value : ['auto'],
   choices : [
-    'auto', 'x11', 'wayland', 'haiku', 'android', 'windows',
+    'auto', 'x11', 'wayland', 'haiku', 'android', 'windows', 'macos',
   ],
   description : 'window systems to support. If this is set to `auto`, all ' +
                 'platforms applicable will be enabled.'