AGOS: Give each MIDI track its own loop flag

This is needed because the MIDI resources have their own loop flag
that overrides the global one. This makes a difference for games
that use MIDI both for music and sound effects, such as (limited
to?) the floppy version of Simon the Sorcerer 1.
Merge pull request #526 from eriktorbjorn/agos-midiloop

AGOS: Give each MIDI track its own loop flag (bug #6665)