光照贴图的数据。
A Scene can have several lightmaps stored in it, and Renderer components can use 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.
另请参阅：LightmapSettings 类、Renderer.lightmapIndex
| lightmapColor | 存储入射光颜色的光照贴图。 |
| lightmapDir | 存储入射光主要方向的光照贴图。 |
| shadowMask | 存储每个光源的遮挡遮罩的纹理（ShadowMask，最多四个光源）。 |