onscreen/native: Forget the view pointer when it is detached
Otherwise it would linger after a hotplug event, pointing to the old (destroyed) view. And it's much easier to debug a NULL pointer than a pointer to freed memory.
I actually wanted to use g_object_add_weak_pointer
in meta_onscreen_native_set_view
but that doesn't seem to work. The weak pointer always comes out NULL. Almost like the view
object is created as a floating reference?