surface: Don't inline z_reorder().

Haxe 3.1 doesn't seem to like this anymore, but haven't investigated
further why exactly.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
object: Mark surface property nullable.

It's actually checked against null/not-null.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>