Fix Object::cast_to<T>() for custom classes (reverted)

[dsnopek]

Fixes #995 and godotengine/godot#71332

I tried a bunch of different variations on this, including using p_object->is_class(class_name) to check if the object was the right type, AND making changes in the Godot engine itself in gdextension_object_cast_to() (which would break the GDExtension ABI, but Remi actually said that would be fine while GDExtension is experimental :-)).

However, assuming there isn't some case where this won't work that I didn't think of, I like the version in this PR the best so far!

It's switching to dynamic_cast<T> at the end, which means C++ will check if the final class matches the requested type using RTTI. This should be much faster than p_object->is_class(class_name) (since that relies on walking up the inheritance chain and comparing strings).

Anyway, it's working in my testing with SG Physics 2D!

[dsnopek]

Hm, maybe this needs something a little more. I'm now seeing issues with non-custom classes! I don't yet understand "why" though, since they should also have RTTI information that dynamic_cast<T> can use?

UPDATE: Nevermind, I think this is separate issue because it's only happening when getting an Object * in and out of Variant - probably another bug! :-)

[mihe]

Seeing as how RTTI is right up there with exception handling in terms of hard lines that divide the C++ ecosystem, I'm curious what the stance of godot-cpp is when it comes to allowing the disabling of RTTI. The code in this PR would, as far as I can tell, be the only thing that would require it to be enabled.

There is one usage of typeid in rid_owner.hpp (which I would consider optional) that seems to allow RTTI to be opt-out if you define NO_SAFE_CAST, which I weirdly can't seem to find any mention of anywhere else.

godot-cpp/include/godot_cpp/templates/rid_owner.hpp

Lines 317 to 321 in f9ccf28

	#ifdef NO_SAFE_CAST
	printf("ERROR: %d RID allocations of type 'unknown' were leaked at exit.", alloc_count);
	#else
	printf("ERROR: %d RID allocations of type '%s' were leaked at exit.", alloc_count, typeid(T).name());
	#endif

Anyway, don't take this as a review or anything. I just happened to stumble upon your PR and it caught my attention. It might very well be that your proposed solution is the only reasonable way of solving this problem.

[akien-mga]

NO_SAFE_CAST was used in Godot 3.x, and it used to be possible to disable RTTI.
For 4.0, RTTI is required on all platforms and NO_SAFE_CAST was removed. Its usage in rid_owner.hpp here seems to show that this file is out of sync with upstream.

Likewise, object.h in upstream Godot does use dynamic_cast nowadays (but this used to be different).

[akien-mga]

Thanks!