Use double-sided material for RayCast3D

This makes RayCast3Ds visible if the camera is fully inside one
(e.g. a RayCast3d parented to the current Camera3D).

scene/3d/ray_cast_3d.cpp

@@ -419,6 +419,8 @@ void RayCast3D::_update_debug_shape_material(bool p_check_collision) {
 		debug_material = material;
 
 		material->set_shading_mode(StandardMaterial3D::SHADING_MODE_UNSHADED);
+		// Use double-sided rendering so that the RayCast can be seen if the camera is inside.
+		material->set_cull_mode(BaseMaterial3D::CULL_DISABLED);
 		material->set_transparency(BaseMaterial3D::TRANSPARENCY_ALPHA);
 	}