reset meters only when *really* necessary

reset meters only when *really* necessary

..and continue to calculate fall-off in
audio-cycle (rather than UI thread)

TODO: check if this works properly when switching
between audio/midi meter modes on a midi-track.

One of the motivations to always reset meters when the
meter-point changes was to resolve peak-hold & fall-off
issues when a midi-meter replaces an audio-meter and vice
versa.