wayland/cursor-surface: Update cursor on dispose

Otherwise we'll have a cursor sprite backed by a surface that no longer exist. This usually doesn't happen, but can happen in rare situations related to pointer capability changes Wayland client cursor changes and hotplugs.

Fixes the following crash:

  #0  meta_wayland_buffer_get_resource() at ../src/wayland/meta-wayland-buffer.c:128
  #1  realize_cursor_sprite_from_wl_buffer_for_gpu() at ../src/backends/native/meta-cursor-renderer-native.c:1649
  #2  realize_cursor_sprite_for_gpu() at ../src/backends/native/meta-cursor-renderer-native.c:1869
  #3  realize_cursor_sprite() at ../src/backends/native/meta-cursor-renderer-native.c:1887
  #4  meta_cursor_renderer_native_update_cursor() at ../src/backends/native/meta-cursor-renderer-native.c:1100
  #5  meta_cursor_renderer_update_cursor() at ../src/backends/meta-cursor-renderer.c:414
  #6  meta_cursor_renderer_force_update() at ../src/backends/meta-cursor-renderer.c:449
  #7  update_cursors() at ../src/backends/meta-backend.c:328
  #8  meta_backend_monitors_changed() at ../src/backends/meta-backend.c:338
  #9  meta_monitor_manager_notify_monitors_changed() at ../src/backends/meta-monitor-manager.c:3590
 #10 meta_monitor_manager_rebuild() at ../src/backends/meta-monitor-manager.c:3678
 #11 meta_monitor_manager_native_apply_monitors_config() at ../src/backends/native/meta-monitor-manager-native.c:343
 #12 meta_monitor_manager_apply_monitors_config() at ../src/backends/meta-monitor-manager.c:706
 #13 meta_monitor_manager_ensure_configured() at ../src/backends/meta-monitor-manager.c:779
 #14 meta_monitor_manager_reconfigure() at ../src/backends/meta-monitor-manager.c:3738
 #15 meta_monitor_manager_reload() at ../src/backends/meta-monitor-manager.c:3745

or the following on gnome-43:

 #0 meta_wayland_surface_get_buffer at ../src/wayland/meta-wayland-surface.c:441
 #1 meta_cursor_sprite_wayland_get_buffer at ../src/wayland/meta-cursor-sprite-wayland.c:83
 #2 realize_cursor_sprite_from_wl_buffer_for_gpu at ../src/backends/native/meta-cursor-renderer-native.c:1612
 #3 realize_cursor_sprite_for_gpu at ../src/backends/native/meta-cursor-renderer-native.c:1836
 #4 realize_cursor_sprite at ../src/backends/native/meta-cursor-renderer-native.c:1854
 #5 meta_cursor_renderer_native_update_cursor at ../src/backends/native/meta-cursor-renderer-native.c:1087
 #6 meta_cursor_renderer_update_cursor at ../src/backends/meta-cursor-renderer.c:413
 #7 meta_cursor_renderer_force_update at ../src/backends/meta-cursor-renderer.c:448
 #8 update_cursors at ../src/backends/meta-backend.c:344
 #9 meta_backend_monitors_changed at ../src/backends/meta-backend.c:354

Related: https://bugzilla.redhat.com/show_bug.cgi?id=2185113