Apply late reverb density gain on late reverb input

Apply late reverb density gain on late reverb input

This also uses gain stepping for changes in density or decay, so that the
late reverb tap fades smoothly between delay offsets and density gain levels
simultaneously. Now with these changes, it's preferrable to apply density gain
adjustments on late reverb input instead of output so that samples currently in
the feedback loop won't see a sudden increase or decrease in amplitude.

A similar change can probably be made for the early reflection delays to
further smooth out delay changes.