This is needed to properly support polyphonic modulation, since the
modulated value may still be automated in the meantime and the
polyphonic modulation must act as an offset for that value. This does
mean that the plugin must add the normalized value and normaliezd offset
by itself. The `PolyModulation` event now also contains a description of
how this can be used. It would have been nicer to be able to send
polyphonic automation-style events instead (like the PolyModulation was
doing before), but that's sadly not feasible without NIH-plug being
involved in the voice management.
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