title: wmts (GeoTiff) for a Mapbox-GL source
body: <p>I am trying to use a wmts (from GeoServer of a GeoTiff) for a Mapbox-GL source. Mapbox-GL is able to create the source and the layer without any error. However, the layer is not rendered, and GeoServer is never queried for the tiles.</p>
  
  <pre><code>map.on('load', function() {
  
      // Create raster layer from GeoServer
      map.addSource('rasterTest', {
          'type': 'raster',
          'tiles': 'http://localhost:32769/geoserver/gwc/service/wmts?SERVICE=WMTS&amp;REQUEST=GetTile&amp;LAYER=enview:sample&amp;TILEMATRIX=EPSG:900913:{z}&amp;TILEMATRIXSET=EPSG:900913&amp;format=image%2Fpng&amp;TileCol={x}&amp;TileRow={y}',
          'tileSize': 256,
      });
  
      map.addLayer({
          'id':1,
          'source': 'rasterTest',
          'type': 'raster',
          'visibility': 'visible',
          'source-layer': 'rasterTest',
      });
  
      console.log('map:');
      console.log(map);
  
  })
  </code></pre>