Make DTS probe more robust against false positives (as e.g. probetest shows).
In particular check that the detected markers clearly indicate a specific DTS
format (a wild mixture of e.g. little- and big-endian markers is unlikely to be
a valid DTS file) and ensure the markers appear with sufficient frequency.

Originally committed as revision 19844 to svn://svn.ffmpeg.org/ffmpeg/trunk
