Zooming threshold code

[jlgrall]

In your threshold code, you refuse values that exceed the threshold:

if (matrix.d >= threshold[1]) {
  return;
}

This code can cause problems if:

• the mousewheel increases the zoom with large steps. For example: start=0, step=30, max=59. You will never get close to 59, you will be limited to 30.
• if I programmatically directly modify the transform attribute to apply a zoom that goes over the threshold, then I cannot zoom back with the mousewheel, it will be completely blocked.
• if you rotate the view, then matrix.d will not be your zoom factor anymore, which makes all your computations wrong. See matrix descriptions in: http://www.w3.org/TR/SVG/coords.html#EstablishingANewUserSpace

[hueitan]

@jlgrall You are right on this. We need a better try to handle zoom threshold.

[hueitan]

Any good idea for this @jlgrall ?