wayland: implement scaling

This involves many subsystems:

- config:
    - allow setting the connector scale
    - allow setting the cursor size
- cursors:
    - load server cursors for all requested sizes and scales
- wl_surface:
    - track the output the surface belongs to
    - send wl_surface.enter/leave
- wl_output:
    - implement wl_output.scale
- text:
    - pre-render texts for all used scales
- renderer:
    - properly align scale textures and rectangles
- wp_fractional_scale:
    - new interface for fractional scaling

src/tree/toplevel.rs

@@ -129,6 +129,11 @@ pub trait ToplevelNode: Node {
     fn tl_set_workspace(self: Rc<Self>, ws: &Rc<WorkspaceNode>) {
         let data = self.tl_data();
         data.workspace.set(Some(ws.clone()));
+        self.tl_set_workspace_ext(ws);
+    }
+
+    fn tl_set_workspace_ext(self: Rc<Self>, ws: &Rc<WorkspaceNode>) {
+        let _ = ws;
     }
 
     fn tl_change_extents(self: Rc<Self>, rect: &Rect) {