Function gl::ReadnPixels

pub unsafe fn ReadnPixels(x: GLint,
                          y: GLint,
                          width: GLsizei,
                          height: GLsizei,
                          format: GLenum,
                          type_: GLenum,
                          bufSize: GLsizei,
                          data: *mut c_void)

Fallbacks: ReadnPixelsARB, ReadnPixelsEXT, ReadnPixelsKHR