Godot may call property setters from non-main thread when an object is
loaded in the edtior. This means NativeScriptLanguage could be accessed
from different threads, but it was not designed for thread-safety.
Besides, previous behaviour made it so that godot_nativescript_init and
godot_gdnative_init could be invoked from non-main thread, while
godot_gdnative_thread is always invoked on the main thread. This may
not be expected by the binding library.
This commit defers native library initialization to the main thread and
adds godot_nativescript_thread_enter and godot_nativescript_thread_exit
callbacks to make a binding library aware of foreign threads.
7f32023a1a