This only removes 2 bytes from mp3 and mp2 currently.
Up to 4 could be removed from mp3/mp2 though this might need a 2pass muxer.
Prmitiv code to remove headers from mpeg1/2/4 is there too but for the
single file i tried it for (the one in the regression tests) it was a loss
because all video frames where >4096 byte, so that its disabled ATM.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@11936 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b