This avoids doing the duplicate check (since both `setProcessing()` and
`setActive()` would be called), and this also gets rid of the assumption
added a couple commits ago that `setupProcessing()` is only ever called
once, which is not true.
This makes much more sense, since all plugin instances will be sharing a
single GUI thread. What would happen was that resize calls from one
instance and GUI thread function calls from another instance would
collide. Using a single shared mutual recursion mechanism (just like on
the Wine side) fixes this.
We can now use implement all VST3 plugin interfaces through this class,
check whether the object from the plugin also supports these classes,
and then conditionally allow casting to the supported classes. This
should give us a one-to-one proxy of the original object.
We're also going to need this for `IEditController`. Separating all of
these classes will also keep everything much more maintainable with all
of these associated structs.
Since it does something way more involved than
`SocketHandler::{send,receive_multi}`, and that makes it a bit confusing
if you don't already know about that (and even if you do).
Robbert van der Helm