Only play the active track when mixing.

@niffur called this 'solo mode'. When the finger goes down, zero all
tracks but the track that's being manipulated. `touchend` triggers all
tracks at the right volume.
Damp other sounds instead of zero`ing them
Damp extra when the current track is quiet

When adjusting the level of a track at <37.5% volume, damp to 5%.
Otherwise 12.5%. Random numbers!
Merge the low hanging fruit of @niffur's design

No fancy scrolling, TODO.

I took a few liberties. Drop the credits to the bottom. Wide screens get
a grid of images.