ModelImporter 的法线生成选项。
Normals can either be imported from the Model file (default), calculated by Unity, or not included in the imported Mesh at all. Vertex normals are most often used for real-time lighting.
另请参阅：ModelImporter.importNormals、ModelImporter.importBlendshapeNormals、ModelImporterTangents、Mesh.normals。