Here's a second cleanup patch for the graphic subsystem. It aims at
definitively solving the potential race conditions which may arise when the
windowed drivers are starting. I've removed all the hooks called from
wwnd.c: now the driver-specific calls are properly encapsulated into a
dedicated interface, which is exposed to the system only after the graphic
driver has completely set up.
The new interface (struct WIN_GFX_DRIVER) is defined in aintwin.h; only the
two DirectDraw windowed drivers currently use it. It can be easily expanded
to support more features (e.g desktop color depth change on the fly).