This depends on upgrading Castle Game Engine class (TCastleWindowBase) to GTK3. See my comments in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=967284 -- we will do it :)
Once the upgrade to GTK3 in Castle Game Engine is done, then view3dscene will just be able to use it, without any additional work. Regards, Michalis
