Fixed minor Godot 3 bit in "gui_in_3d" demo's billboard specific code

[Tarik-B]

• Godot 3's SpatialMaterial has a "params_billboard_mode" property: https://docs.godotengine.org/en/3.6/classes/class_spatialmaterial.html#class-spatialmaterial-property-params-billboard-mode
• but Godot 4's BaseMaterial3D instead has a "billboard_mode" property: https://docs.godotengine.org/en/stable/classes/class_basematerial3d.html#class-basematerial3d-property-billboard-mode

But I guess nobody needed any convincing since that change has already been made at line 20: if node_quad.get_surface_override_material(0).billboard_mode == BaseMaterial3D.BillboardMode.BILLBOARD_DISABLED:

[Tarik-B]

Thanks, good catch! Didn't test but I assume this works.

Well I did test it since, as you can imagine, I stumbled upon it while trying the demo.