wlr_drag: emit destroy after wl_data_device.leave

In my compositor, I currently may trigger a wl_pointer.enter event to be sent from the wlr_drag.destroy signal handler. This seems to confuse at least firefox and the "Tool Palette" gtk3 demo example to the point that they fail to handle pointer input correctly until the next wl_pointer.leave event. I've been pointing fingers at river, gtk, and wlroots during this debugging session but I'm pretty sure the fix belongs here in wlroots.