title: Get canvas current scale
body: <p>I am currently working with a canvas that holds shapes. So I want to be able to keep the shapes size while zooming in or out. 
  I think that I need to know the canvas scale or the dpi, so I can recalculate the shape dimensions. My questions</p>
  
  <ul>
  <li>Is there better solution for that?</li>
  <li>If I am on the good track how can I get the current scale of a canvas? I am looking for something like in the code below. 
  <code>let scale = ctx.getCurrentScale();</code>.</li>
  </ul>
  
  <p>Thanks for your help.</p>