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_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

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2969>