Disable image smoothing for the DEM source

examples/sea-level.html

@@ -8,6 +8,11 @@ docs: >
     <a href="https://www.mapbox.com/blog/terrain-rgb/">Mapbox Terrain-RGB tiles</a>
     to "flood" areas below the elevation shown on the sea level slider.
   </p>
+  <p>
+    <code>ol/source/Raster</code> can take either a tile source or layer.
+    In this case a layer is used to allow disabling at the <code>prerender</code> event
+    of image smoothing which would change the precise elevation values set in the pixels.
+  </p>
 tags: "raster, pixel operation, flood"
 cloak:
   - key: pk.eyJ1IjoidHNjaGF1YiIsImEiOiJjaW5zYW5lNHkxMTNmdWttM3JyOHZtMmNtIn0.CDIBD8H-G2Gf-cPkIuWtRg