mp3demux: Rewrite xing TOC based seeking

The libav code depends on mp3 startcodes only occuring at the start of
frames. But there is nothing in mp3 that prevents them occuring elsewhere
by chance. Thus the code would fail randomly, the new code searches for 3
consecutive valid frames. If this turns out to be still insufficient the
number can be raised further, or additional checks added.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
