client API: reasonable behavior if window is closed

Closing the video window sends CLOSE_WIN, which is normally mapped to
the "quit" command. The client API normally disables all key bindings,
and closing the window does nothing. It's simply left to the application
to handle this. This is fine - an embedded window can not be destroyed
by user interaction.

But sometimes, the window might be destroyed anyway, for example because
the containing window is destroyed. If this happens, CLOSE_WIN should
better not be ignored. We can't expect client API users to handle this
specially (by providing their own input.conf), so provide some fallback
for this pseudo key binding. The "quit" command might be too intrusive
(not every client necessarily handles "unexpected" MPV_EVENT_SHUTDOWN),
but I think it's still reasonable.

input/input.c

@@ -460,6 +460,8 @@ static mp_cmd_t *get_cmd_from_keys(struct input_ctx *ictx, char *force_section,
 
     struct cmd_bind *cmd = find_any_bind_for_key(ictx, force_section, code);
     if (cmd == NULL) {
+        if (code == MP_KEY_CLOSE_WIN)
+            return mp_input_parse_cmd_strv(ictx->log, (const char*[]){"quit", 0});
         int msgl = MSGL_WARN;
         if (code == MP_KEY_MOUSE_MOVE || code == MP_KEY_MOUSE_LEAVE)
             msgl = MSGL_DEBUG;