* Added double-buffering for the directx plugin in overlay mode.
* Several directx bug-fixes:
    - implemented the right behaviour on exit.
    - fixed bug preventing the creation of several windows.
        (the wall filter is working now :-))
    - fixed unnecessary background redraws (it made the video flicker).
    - fixed bug where we were losing the video display because of lost
        surfaces in video memory.
    - when using hardware overlay, the output window will now specify:
        "(using hardware overlay)".