2. void glGenerateMipmap(    GLenum target);
    4. // 为target 生成mipmaps（多级纹理）
    6. // target:    可能值如下：
    7. //        GL_TEXTURE_1D, 
    8. //        GL_TEXTURE_2D, 
    9. //        GL_TEXTURE_3D, 
    10. //        GL_TEXTURE_1D_ARRAY, 
    11. //        GL_TEXTURE_2D_ARRAY 
    12. //        GL_TEXTURE_CUBE_MAP.

    GL_INVALID_ENUM is generated if target is not one of the accepted texture targets.
    GL_INVALID_OPERATION is generated if target is GL_TEXTURE_CUBE_MAP and the texture bound to the GL_TEXTURE_CUBE_MAP target of the active texture unit is not cube complete.