removed video capture formats other than AVI I420 as it is the only useful format...

removed video capture formats other than AVI I420 as it is the only useful format in practice (lots of jpeg/tga files is annoying to deal with, and few programs support rgb/yv12 raw dumps), this means no more .wav saving either and cleans the code a bit
fixed several bugs in sound system when saving avi (lingering .wav code)
added cl_capturevideo_realtime cvar to allow realtime capture if desired (requires a fast machine and can have A/V sync issues)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6590 d7cf8633-e32d-0410-b094-e92efae38249