This got removed in 74dc8225d1 when I
wanted to rework all `*Impl` classes the same way I did with
`YaPluginFactoryImpl`. This ended up not being possible, but
accidentally also removed this forward declaration. With unity builds
this did not cause issues however, but with regular builds it might
depending on which files are changed.
This prevents REAPER from crashing when removing the last instance of a
plugin and then readding it. REAPER doesn't unload the module even after
it removes its last plugin factory instance. This means that before this
the plugin factory would be freed but we still had a seemingly valid
pointer to it that we would try to access.
This way every relevant object instance will get its own thread for
handling these calls. The alternative would be creating a full fat
Vst3MessageHandler pair for all object instances, but that would be a
huge waste.
As it turns out there are only two or three functions where we can do
this. It also breaks logging, and this function will probably only be
called once anyways. More consistency is always better.
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.
Since the object cleans up after itself after the smart pointers are
dropped on the host side this would result in a use after free by the
smart pointers.
Robbert van der Helm