fixed several crashes in video capture code when files fail to open

fixed several crashes in video capture code when files fail to open
eliminated cl_capturevideo_sound cvar (now always on)
made sound capture non-realtime, by syncing sound mixer to capture frame while capture is active, this sounds horrible while recording but produces perfectly synced sound output to the .wav file
now numbers dpvideo files when saving, each one a higher number than the previous (the next number to use is saved to config), this allows capturing many short video clips without stopping to process the files
what this means: cl_capturevideo is now much more useful than before.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6408 d7cf8633-e32d-0410-b094-e92efae38249