Adds a vertex attribute input called 'a_vertexId' into this sub-mesh.
This is useful if you want to simulate gl_VertexId in WebGL context prior to 2.0.
Once you call this function, the vertex attribute is permanently added.
Subsequent calls to this function take no effect.
包含所有顶点数据的渲染子网格,可以用来创建 GFXInputAssembler。