Properly fix bugs #3267956 and #3605377

SCI: Only stop after fading, if the song to be faded is faded down. Also,
reset the song signal when fading starts. It was set to -1 when fading
started in bug #3267956, thus it stopped immediately.
SCI: Add a hack to fix bug #3596335

Game scripts are waiting indefinitely for a song, thus we change its dataInc
selector to prevent that from happening