title: get touchstart event to bubble up from d3 zoom to the svg
body: <p>I am using the d3 zoom behaviour on my svg element and i also have a dblclick event of the svg element.</p>
  
  <p>Under desktop i can zoom with mouse wheel and double click as needed. However when i go to an ipad the double click does not go through. </p>
  
  <p>So i have added a touchstart listener to handle a double tap on the svg (the svg also has the zoom behaviour associated with it)</p>
  
  <p>The trouble i am getting is that the zoom appears to grab the touch start and then the underlining svg touchstart event does not go through so i cannot do my double tap.</p>
  
  <p>Is there a way to keep the touchstart in the zoom, as it is needed for the paning and pinch zooming i believe, as if i do .on('touchstart.zoom',null) panning does not work and pinch zoom does not work either. But allow me to bubble it up so i can do my double tap functionality.</p>
  
  <p>Cheers
  Mark</p>
  
  <p><strong>UPDATE- Looking into this a bit more the W3C standard dictate that this is the way it should work and touch events are to be stopped from bubbling up</strong></p>