* modules/audio_output/waveout.c: compilation fix for --disable-plugins build.
* src/video_output/*, include/video.h: added a pf_lock/unlock fonction
   pointer that's used to lock the memory of a picture in vout_RenderPicture
   when required by the video output plugin.
* modules/video_output/directx/*: implemented pf_lock/unlock. That will fix
   the crash when pressing ctrl+alt+del.