embedding custom wayland UI stuff on top of gtk3 and gtk4 based applications works! 🎉

the gtk APIs to get down to the wayland surface are a bit awkward, but at least its accessible.
I tried the same with Qt6 and couldn't find a way to have it return the underlying wl_surface pointer. 🤷

I got an LV2 plugin UI with this setup working, but no hosts to try it out against.
Modifying jalv.gtk3 is likely the easiest path... 🤔

WIP test code dump at https://github.com/falkTX/wayland-audio-plugin-test