[Backport to 3.38] backend: Keep a refcount on backend's current_device

Otherwise we might run into a use-after-free and crash on (virtual) device removal:

Invalid read of size 8 at clutter_input_device_get_device_type (clutter-input-device.c:811) by update_last_device (meta-backend.c:1282) by g_main_dispatch (gmain.c:3325) by g_main_context_dispatch (gmain.c:4016) by g_main_context_iterate.constprop.0 (gmain.c:4092) by g_main_loop_run (gmain.c:4290) by meta_run_main_loop (main.c:708) by meta_run (main.c:723) by main (main.c:550) Address is 32 bytes inside a block of size 504 free'd at free (vg_replace_malloc.c:538) by g_type_free_instance (gtype.c:1939) by clutter_event_free (clutter-event.c:1420) by _clutter_stage_process_queued_events (clutter-stage.c:830) by handle_frame_clock_before_frame (clutter-stage-view.c:1064) by clutter_frame_clock_dispatch (clutter-frame-clock.c:405) by frame_clock_source_dispatch (clutter-frame-clock.c:456) by g_main_dispatch (gmain.c:3325) by g_main_context_dispatch (gmain.c:4016) by g_main_context_iterate.constprop.0 (gmain.c:4092) by g_main_loop_run (gmain.c:4290) by meta_run_main_loop (main.c:708) by meta_run (main.c:723) Block was alloc'd at at malloc (vg_replace_malloc.c:307) by g_malloc (gmem.c:106) by g_slice_alloc (gslice.c:1025) by g_slice_alloc0 (gslice.c:1051) by g_type_create_instance (gtype.c:1839) by g_object_new_internal (gobject.c:1939) by g_object_new_valist (gobject.c:2264) by g_object_new (gobject.c:1782) by meta_input_device_native_new_virtual (meta-input-device-native.c:1365) by meta_virtual_input_device_native_constructed (meta-virtual-input-device-native.c:705) by g_object_new_internal (gobject.c:1979) by g_object_new_valist (gobject.c:2264)

Suggested-by: Carlos Garnacho carlosg@gnome.org !1529 (merged) (cherry picked from commit 8711d8d5)