e canvas2d example if off = 0 then the image will just
  // be flattly stretched down the canvas. "off" is an offset in
  // texture coordinates of which part of the source image to use
  // for the current destination. 
 
  // In the canvas example off was in pixels since in +1 means use the
  // src image 1 pixel lower than we would have used and -1 = one pixel higher

  // In shaders we work in texture coords which go from 0 to 1 regardless
  // of the size of the texture. So for example if the texture w