wayland: set current_output on surface leave if applicable

When the mpv surface leaves the output, we no longer mark it as the
current output. However, this implicitly depends on there being a
preceding surface entrance event to a different output. This is not
necessarily the case. Consider moving the window from monitor 1, to
monitor 1-2, and then back to 1 again. mpv gets the entrance event to
monitor 2 and sets that as the current output to work off of. Then when
you move back to only monitor 1, it removes monitor 2 from the current
output. However, monitor 1 is not updated again as the current output
because there is not a new surface entrance event in this case (the
window never left). So the numbers that mpv's core is using are
incorrect and for the wrong monitor. Luckily, we already keep track of
what outputs the mpv surface is currently on no matter how many there
are so it is simply a matter of setting current output again in the
leave event if we have a different output that has the mpv surface.

Ref: swaywm/sway#7932

video/out/wayland_common.c

@@ -904,12 +904,12 @@ static void surface_handle_leave(void *data, struct wl_surface *wl_surface,
 
     struct vo_wayland_output *o;
     wl_list_for_each(o, &wl->output_list, link) {
-        if (o->output == output) {
+        if (o->output == output)
             o->has_surface = false;
-            wl->pending_vo_events |= VO_EVENT_WIN_STATE;
-            return;
-        }
+        if (o->output != output && o->has_surface)
+            wl->current_output = o;
     }
+    wl->pending_vo_events |= VO_EVENT_WIN_STATE;
 }
 
 #ifdef HAVE_WAYLAND_1_22