Fixed the sounds not being advanced in time while not being listened. This is a long...

Fixed the sounds not being advanced in time while not being listened. This is a long standing bug I think, it looks like the original Quake source code has it too, though I haven't tested. Each channel now remembers the last time it was painted, and so it can resume the play at the appropriate position if it missed some painting rounds.
Also, added a check to avoid looped sounds being overridden when picking a channel.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@4055 d7cf8633-e32d-0410-b094-e92efae38249