solid.mirror: a plane's {origin} takes length expressions and follows them (#1492) (#1501)

Thanks @erkexzcx.

crates/doc/src/assembly.rs

@@ -136,6 +136,11 @@ fn map_plane(m: &Mat, p: &Plane) -> Plane {
 fn map_plane_ref(m: &Mat, p: &mut PlaneRef) {
     match p {
         PlaneRef::Custom { plane } => *plane = map_plane(m, plane),
+        PlaneRef::Placed { origin, x_dir, y_dir } => {
+            map_point_expr(m, origin);
+            *x_dir = apply_vector(m, *x_dir);
+            *y_dir = apply_vector(m, *y_dir);
+        }
         PlaneRef::Face { plane, at, .. } => {
             *plane = map_plane(m, plane);
             *at = apply_point(m, *at);