サブメッシュのインデックスバッファを設定します。
A sub-Mesh is simply a separate indices array. When the Mesh Renderer uses multiple Materials, you should
ensure that there are as many sub-Meshes as Materials.
SetTriangles and ::triangles always set the Mesh to be composed of triangle
faces. Use SetIndices to create a Mesh composed of lines or points.
関連項目: subMeshCount、MeshTopology 列挙体 .