Improve music sequence identification for the two relevant SFX settings

[GSKirox]

The cosmetics settings 'Speed Up Music For Last Triforce Piece' and 'Slow Down Music When Low HP' relied on looking at the seq_index value of the structure z64_file to identify when to apply the change.
This had the drawback of not noticing some specific cases where another music sequence play on top of the current one played in the area, such as the miniboss music in dungeons, the windmill track, or the "Master Sword" track when you switch age in Temple of Time.
Instead, we now use this function : https://github.com/zeldaret/oot/blob/master/src/audio/sequence.c#L410
Which gives the accurate sequence index in every situation, and will correctly apply the music change on all specific cases stated above.