ライトマッピングに使うタイルとオフセット
A scene can have several lightmaps stored in it, and Renderer can use one of those lightmaps.
This makes it possible to use the same material on multiple objects, while
each object can refer to a different lightmap or different portion of the same lightmap.
The vector's x and y refers to the lightmap scale, while z and w refers to the lightmap offset.
See Also: LightmapSettings クラス, lightmapIndex プロパティ, ShaderLab プロパティ.