* 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)".