Proper initialization for godot-cpp classes with memnew.
Extension classes (i.e. the `GDCLASS` macro) behave differently from
regular wrapped classes, and requires Godot to initialize them during
object construction.
This commit update the GDCLASS macro to not create/destroy the instance
during the bindings callback, but during the extension callbacks.
When setting the object instance, the bindings instance is set to the
pointer of the extension instance so that it can later be retrieved
normally via `object_get_instance_bindings`.
Fabio Alessandrelli
Bastiaan Olij
George Marques