KYRA: Fix MIDI fade-out behaviour

This takes a bit of explaining...

I noticed that when MIDI music is faded out, the volume first gets louder than before. The simplest way I could find to hear this is to start the Kyrandia 2 intro while the main menu music is still playing.

Here is what I think is happening:

There are two ways that the Kyra MIDI player sets the music:

1) The setSourceVolume() function.
2) Volume events, handled by the MIDI parser. These are caught by send().

Music fade-out is handled by calling setSourceVolume() repeatedly, with smaller and smaller volume.

Note that calling sendIntern() bypasses the send() function.

As I understand it, setSourceVolume() is used to set the music volume as ScummVM sees it. This is stored in _sources[].volume.

The send() function handled music volume as the MIDI parser sees it. This volume is usually (always?) 100, but let's assume it's not. This volume is stored in _sources[].controllers[][].value. Before it's sent to the actual MIDI backend, it's adjusted by what is in _sources[].volume.

So what setSourceVolume() should do, then, is to adjust _sources[].volume, and set the volume to what it would have been if that had been the value in _sources[].volume when the volume was set in send(). But setSourceVolume() used _channels[].controllers[].value, where I think it should be using _sources[].controllers[][].value. So that's the change I've made here.