tree: implement tile dragging

src/gfx_api.rs

@@ -722,6 +722,9 @@ pub fn create_render_pass(
                     }
                 }
             }
+            if let Some(highlight) = seat.ui_drag_highlight() {
+                renderer.render_highlight(&highlight.move_(-rect.x1(), -rect.y1()));
+            }
             if let Some(drag) = seat.toplevel_drag() {
                 drag.render(&mut renderer, &rect, x, y);
             }