skins2: skin_main: fix window closing after interface

The interface cannot be closed before the vout_window_t object is disabled, since the playlist/player is running from outside the interfaces.

When it happens, a warning was raised from skins2 and the OpenGL code would stay stuck waiting for xcb events, preventing the full closing of the interface resources and delaying the close of the application itself.

This commit ensures that no skins2 window is enabled before leaving the Close function from the interface, and no new skins2 window will be enabled in a racy pattern while or after the Close function from the interface is running.