title: Why does mapshot in R (mapview package) not save the addWebGLHeatmap (leaflet extras) layer?
body: <p>Please help me with the following.</p>
  
  <p>I am making a heatmap using leaflet and addWebGLHeatmap. This works fine, until I try and save the plot as PNG using webshot. See the code below:</p>
  
  <pre><code>m &lt;- map %&gt;% 
    addWebGLHeatmap(data = dataset, lng=~lon, lat=~lat, size = 800, 
    opacity = 0.6, intensity = 1) %&gt;% 
    addPolygons(data = shapefile, weight = 2, color = "black", fillOpacity = 0)
  
  mapshot(m, file = "testRplot.png")
  </code></pre>
  
  <p>What it shows in plotviewer:
  <a href="https://i.stack.imgur.com/EvdrV.jpg" rel="nofollow noreferrer">Plot in viewer</a></p>
  
  <p>What it shows after saving:
  <a href="https://i.stack.imgur.com/sQiqg.jpg" rel="nofollow noreferrer">Plot after saving</a></p>
  
  <p>Hope anyone is able to help.</p>