附加纹理坐标。
UV 通常使用 Vector2 格式。Vector2 使用两个 32 位浮点数；总共 8 个字节。
This vertex attribute maps to the TEXCOORD3 semantic in HLSL. This UV channel is also commonly called "UV3". It corresponds to Mesh.uv4.
Additional resources: Mesh.HasVertexAttribute, Mesh.GetUVs, Mesh.SetUVs.