Prevent exception when drag event target not under svg

[madprog]

🐛 Bug Fix

• In order to have a continuous dragging even if my mouse gets out of the <svg> element, I attach dragMove and dragEnd to the window object in my code.
  However, these events are no longer guaranteed to have a target under a <svg>, provoking an exception in @vx/event:

  vx-event.es.js:15 Uncaught TypeError: Cannot read property 'ownerSVGElement' of undefined
      at localPoint (vx-event.es.js:15)
      at Drag.dragMove (vx-drag.es.js:121)
      at DragViewBase._this.handleMouseMove (DraggableSvg.tsx:75)
  

  This exception is fixed by the changes of this PR.

🏠 Internal

• Extracted outermostSVGElement from localPoint: a function that retrieves the englobing SVG element of a node. localPoint uses it on the event.target if no node was passed, but we may want to use another element as the node and use the same function to retrieve the outermost SVG element.
  For example, the next commit uses it in Drag to always use the outermost SVG element from the g instead of the one from the event, because that event could have a target outside the SVG tree.