src/libvlc.c: Added a quicktime_lock on SYS_DARWIN
* ALL: QT on OSX is not threadsafe in certain areas. i have found the proces
  of identifying the right Codec to use for decoding to be one of these areas.
  So when we do this, only one thread at a time can use QT.
  Watching QT trailers is doable now, as soon as ffmpeg fix their altivec optimizations again ;)