spc: Set a playback length by default if set in header.
spc: Set a playback length by default if set in header.

This is a behavior change -- SPC files that have a valid length set in
their header will default to ending playback once that amount of
playback time has elapsed by using the fadeout function.

Previous behavior was to loop until silence is detected unless a fadeout
time was manually set.

To loop forever, you can manually set the fadeout time to -1 after
starting playback on an SPC track.

ShiftMediaProject/game-music-emu,ShiftMediaProject/game-music-emu,ShiftMediaProject/game-music-emu