Function glCopyTextureSubImage3DEXT 

pub unsafe extern "C" fn glCopyTextureSubImage3DEXT(
    texture: GLuint,
    target: GLenum,
    level: GLint,
    xoffset: GLint,
    yoffset: GLint,
    zoffset: GLint,
    x: GLint,
    y: GLint,
    width: GLsizei,
    height: GLsizei,
)