avformat: move PROBE_BUF_M* to internal.h

They will be used by the mp3 probe function in the next commit

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mp3dec: improve detection of mp3s with huge id3 tags

When a id3 tag is larger than the amount of space we will probe
give it a score similar to what a .mp3 extension would have given it

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