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/backends/metal/video.rs

@@ -248,6 +248,7 @@ impl MetalConnector {
                 Some(node.global.pos.get()),
                 true,
                 &mut rr,
+                node.preferred_scale.get(),
             );
             for fr in rr.frame_requests.drain(..) {
                 fr.send_done();

src/backends/x.rs

@@ -699,6 +699,7 @@ impl XBackend {
                 Some(node.global.pos.get()),
                 true,
                 rr.deref_mut(),
+                node.preferred_scale.get(),
             );
             for fr in rr.frame_requests.drain(..) {
                 fr.send_done();