Add support for ImageTexture3D serialization

[jsjtxietian]

Fixes #80866

In the issue comment, @clayjohn posted a partially working diff to support ImageTexture3D serilization. I took it and modifed the code to make it work correctly (at least in the MRP provided).

If anyone needs this function, feel free to take it and help test this code. I would appreciate any feedback or alternative suggestions.

[bitsawer]

Those errors might come when the binary or text resource loader tries to get the property default value (ClassDB::class_get_default_property_value()). Adding a simple test to _get_images() might be enough as we are lazily allocating texture RID's and returning an empty array is fine. So it would become like this:

TypedArray<Image> ImageTexture3D::_get_images() const {
	TypedArray<Image> images;
	if (texture.is_valid()) {
		Vector<Ref<Image>> raw_images = get_data();
		ERR_FAIL_COND_V(raw_images.is_empty(), TypedArray<Image>());

		for (int i = 0; i < raw_images.size(); i++) {
			images.push_back(raw_images[i]);
		}
	}
	return images;
}

ImageTextureLayered might possibly need a similar check if used in a same manner as the ImageTexture3D in the issue code snippet, but that would need some more testing and is a bit out of scope of this PR.

Kind of speculating here, so needs testing for confirmation.

[jsjtxietian]

Thanks! It solved my problem.

[bitsawer]

Tested locally on top of current master (9e455f4), seems to work and fixes the linked issue and serialization.

@clayjohn might still want to take a final look as it is mostly his suggested code.

[clayjohn]

Looks fine to me. This isn't really my area of expertise, but I also tested locally and it appears to work fine.

[akien-mga]

Thanks!