Previously, all VST3 parameter changes were signalled to the host via
performEdit, regardless of the originating thread. This breaks the
contract of the IEditController interface, which requires that all calls
are made from the UI thread.

This change modifies the VST3 wrapper so that it will only call
performEdit when a parameter is set on the message thread. If a
parameter is set on the audio thread instead, then the parameter change
will be signalled to the host using
`ProcessData::outputParameterChanges`.

If your VST3 plugin uses a background thread to update parameters for
some reason, you'll encounter data races. Please don't do that.