✨ ensure that context is properly bound

• bind window and document from current context (iframe support).
• bind "this" to computed or relation state element.

@gdeblasi we may not need it for window/document, but if we had more complicated processing to do we could have:

<svg>
  <script>
    window.formatNumber = function formatNumber(value) {
      return (value < 1 && value > -1) ? value.toFixed(2) : value.toPrecision(4);
    }
  </script>
...
  <state>
    <property ...>
      <direct onupdate='formatNumber' />
</svg>

For the this binding it may be used to make our update blinker, ex:

<svg>
  <script>
    window.formatNumber = function formatNumber(value) {
      return (value < 1 && value > -1) ? value.toFixed(2) : value.toPrecision(4);
    }
  </script>
...
  <state>
    <computed name='tick' onupdate='this.tick = !this.tick'>
      <relation query-selector='#tick' attribute-name='opacity' values='0;1;0' />
</svg>

With a strict transition of about 250ms it should do the trick and blink on update (any update).