mp3 header (de)compression bitstream filter

mp3 header (de)compression bitstream filter
this will make mp3 frames 4 bytes smaller, it will not give you binary identical mp3 files, but it will give you mp3 files which decode to binary identical output
this will only work in containers providing at least packet size, sample_rate and number of channels
bugreports about mp3 files for which this fails are welcome
and this is experimental (dont expect compatibility and dont even expect to be able to decompress what you compressed, hell dont even expect this to work without editing the source a little)

Originally committed as revision 6958 to svn://svn.ffmpeg.org/ffmpeg/trunk