Flat plane shape for use with occlusion culling in [OccluderInstance3D]. [QuadOccluder3D] stores a flat plane shape that can be used by the engine's occlusion culling system. See also [PolygonOccluder3D] if you need to customize the quad's shape. See [OccluderInstance3D]'s documentation for instructions on setting up occlusion culling. The quad's size in 3D units.