title: Animated transition of leaflet Marker icon
body: <p>I have a Leaflet marker with a BLUE icon (from a PNG image source). On mouse hovering, I want to make that a RED icon. </p>
  
  <p>This is the code for doing so:</p>
  
  <pre><code>            let placeMarker = L.marker(...);
  
              let cssSetter = function (imarker, iicon) {
                  imarker.setIcon(iicon);
              };
  
              placeMarker.on('mouseover', 
                  L.bind(cssSetter, null, placeMarker, markerIcon_red)
              );
              placeMarker.on('mouseout', 
                  L.bind(cssSetter, null, placeMarker, markerIcon)
              );
  </code></pre>
  
  <p>Now I want to do this transition gradually, with an animation.</p>
  
  <p>How can I do this?</p>