added cl_avidemo cvar (saves a series of tga images named gamedir/dpavi000000.tga...

added cl_avidemo cvar (saves a series of tga images named gamedir/dpavi000000.tga and so on, the parameter to it is the framerate to save, locks framerate to this while saving, set back to 0 to stop - note there is sound saving code but it is commented out because it's always desynchronized to the video because quake has such a lousy sound engine)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@1693 d7cf8633-e32d-0410-b094-e92efae38249