The number of sub-meshes in the MeshData.
每个子网格都对应于 Renderer 中的一个 Material，如 MeshRenderer 或 SkinnedMeshRenderer。子网格由引用一组顶点的三角形的列表组成。顶点可以在多个子网格之间共享。
If the MeshData is writeable, you can set the number of sub-meshes using this property. If the MeshData is read-only, this property is read-only.
Additional resources: Mesh.subMeshCount, GetSubMesh, SetSubMesh, GetIndices, Mesh.AllocateWritableMeshData, Mesh.AcquireReadOnlyMeshData.