x11: attempt to make initial fullscreening more reliable

It appears some WMs have a problem with out method of setting initial
fullscreen mode. We assume that if the window's _NET_WM_STATE includes
_NET_WM_STATE_FULLSCREEN before mapping the window, the WM will show it
as fullscreen at mapped. EWMH doesn't say anything that this should
work, although one could argue that it's implied.

In any case, since it's not standard behavior without at least some
doubt, it's probably a good idea to try the "old" method as well.
Fortunately, it should be idempotent.

See #1937, #1920.

(cherry picked from commit a4f3df5)

video/out/x11_common.c

@@ -1389,6 +1389,9 @@ static void vo_x11_map_window(struct vo *vo, struct mp_rect rc)
     vo_x11_selectinput_witherr(vo, x11->display, x11->window, events);
     XMapWindow(x11->display, x11->window);
 
+    if (vo->opts->fullscreen && (x11->wm_type & vo_wm_FULLSCREEN))
+        x11_set_ewmh_state(x11, "_NET_WM_STATE_FULLSCREEN", 1);
+
     vo_x11_xembed_update(x11, XEMBED_MAPPED);
 }