wayland: Reuse buffers and pools; check buffer release

Also updates `winit` example to redraw on resize, which seems to be
necessary.

With this resizing seems to be entirely smooth, without visual
corruption from it overwriting the buffer the server is displaying.

examples/winit.rs

@@ -54,6 +54,12 @@ fn main() {
             } if window_id == window.id() => {
                 *control_flow = ControlFlow::Exit;
             }
+            Event::WindowEvent {
+                event: WindowEvent::Resized(_),
+                window_id,
+            } if window_id == window.id() => {
+                window.request_redraw();
+            }
             _ => {}
         }
     });