Pause/stop should ramp

[Ralith]

These probably(?) cause popping as-is.

[mooman219]

I've reproduced it, the code here will need its transition smoothed

oddio/src/mixer.rs

Lines 92 to 98 in adc60db

	if signal.is_stopped() {
	this.set.remove(i);
	continue;
	}
	if signal.is_paused() {
	continue;
	}

[Ralith]

A special case will be required in SpatialScene to support this for Seek signals.