Fixed unbounded dual-paraboloid shadow map culling.

Dual paraboloid shadowmaps were ending up with infinitely large volumes of area behind the hemisphere un-culled.

This change just adds a back plane to the convex shape used for the culling volume.

servers/rendering/rendering_server_scene.cpp

@@ -1850,12 +1850,13 @@ bool RenderingServerScene::_light_instance_update_shadow(Instance *p_instance, c
 
 					real_t z = i == 0 ? -1 : 1;
 					Vector<Plane> planes;
-					planes.resize(5);
+					planes.resize(6);
 					planes.write[0] = light_transform.xform(Plane(Vector3(0, 0, z), radius));
 					planes.write[1] = light_transform.xform(Plane(Vector3(1, 0, z).normalized(), radius));
 					planes.write[2] = light_transform.xform(Plane(Vector3(-1, 0, z).normalized(), radius));
 					planes.write[3] = light_transform.xform(Plane(Vector3(0, 1, z).normalized(), radius));
 					planes.write[4] = light_transform.xform(Plane(Vector3(0, -1, z).normalized(), radius));
+					planes.write[5] = light_transform.xform(Plane(Vector3(0, 0, -z), 0));
 
 					int cull_count = p_scenario->octree.cull_convex(planes, instance_shadow_cull_result, MAX_INSTANCE_CULL, RS::INSTANCE_GEOMETRY_MASK);
 					Plane near_plane(light_transform.origin, light_transform.basis.get_axis(2) * z);