title: SVG Filter with a mask
body: <p>There is a filter</p>
  
  <pre><code>&lt;filter id="filter"&gt;
   &lt;feGaussianBlur stdDeviation="4"&gt;
   &lt;feColorMatrix type="matrix" values=""&gt;
  &lt;/filter&gt;
  </code></pre>
  
  <p>applied to image</p>
  
  <pre><code>&lt;image filter="url('#filter')"&gt;
  </code></pre>
  
  <p>I have a certain shape:</p>
  
  <pre><code>&lt;polygon points=""&gt;
  </code></pre>
  
  <p>How can I use the polygon as a mask where the filter is not applied?</p>