detect_music_type() does not detect valid MPEG Audio files

[Fulgen301]

#288 reported missing support for an MPEG Audio frame header starting with the first eleven bits. However, an MPEG file could start anywhere (e.g. in the middle of a frame) and audio players should seek to the first occurence of those frame sync bits and check whether the header is valid.

I encountered this issue when I was informed that certain MPEG-1 files weren't loaded by Mix_LoadWAV_RW with Unrecognized audio format. The same audio files are played by mpg123 and vlc without issues, so while the source for this behavior isn't an official document, it seems to be the case that MPEG Audio files are in fact allowed to start anywhere.

[slouken]

Can you provide a link to an example file that doesn't load correctly?

[Fulgen301]

Better late than never, sorry about that: https://fulgen.at/files/011.wav (it has the wrong file extension as it contains MPEG data, but SDL_mixer doesn't check them anyway).