Remove the bulk of the WebGL legacy code.

Things left to do:
* redo an icon layer example
* redo a clipping layer example
* update docs where WebGL renderers are mentioned

examples/icon-sprite-webgl.js

@@ -1,136 +0,0 @@
-import Feature from '../src/ol/Feature.js';
-import Map from '../src/ol/WebGLMap.js';
-import View from '../src/ol/View.js';
-import Point from '../src/ol/geom/Point.js';
-import VectorLayer from '../src/ol/layer/WebGLVector.js';
-import VectorSource from '../src/ol/source/Vector.js';
-import {Icon, Style} from '../src/ol/style.js';
-
-
-const iconInfo = [{
-  offset: [0, 0],
-  opacity: 1.0,
-  rotateWithView: true,
-  rotation: 0.0,
-  scale: 1.0,
-  size: [55, 55]
-}, {
-  offset: [110, 86],
-  opacity: 0.75,
-  rotateWithView: false,
-  rotation: Math.PI / 2.0,
-  scale: 1.25,
-  size: [55, 55]
-}, {
-  offset: [55, 0],
-  opacity: 0.5,
-  rotateWithView: true,
-  rotation: Math.PI / 3.0,
-  scale: 1.5,
-  size: [55, 86]
-}, {
-  offset: [212, 0],
-  opacity: 1.0,
-  rotateWithView: true,
-  rotation: 0.0,
-  scale: 1.0,
-  size: [44, 44]
-}];
-
-let i;
-
-const iconCount = iconInfo.length;
-const icons = new Array(iconCount);
-for (i = 0; i < iconCount; ++i) {
-  const info = iconInfo[i];
-  icons[i] = new Icon({
-    offset: info.offset,
-    opacity: info.opacity,
-    rotateWithView: info.rotateWithView,
-    rotation: info.rotation,
-    scale: info.scale,
-    size: info.size,
-    crossOrigin: 'anonymous',
-    src: 'data/Butterfly.png'
-  });
-}
-
-const featureCount = 50000;
-const features = new Array(featureCount);
-let feature, geometry;
-const e = 25000000;
-for (i = 0; i < featureCount; ++i) {
-  geometry = new Point(
-    [2 * e * Math.random() - e, 2 * e * Math.random() - e]);
-  feature = new Feature(geometry);
-  feature.setStyle(
-    new Style({
-      image: icons[i % (iconCount - 1)]
-    })
-  );
-  features[i] = feature;
-}
-
-const vectorSource = new VectorSource({
-  features: features
-});
-const vector = new VectorLayer({
-  source: vectorSource
-});
-
-const map = new Map({
-  layers: [vector],
-  target: document.getElementById('map'),
-  view: new View({
-    center: [0, 0],
-    zoom: 5
-  })
-});
-
-const overlayFeatures = [];
-for (i = 0; i < featureCount; i += 30) {
-  const clone = features[i].clone();
-  clone.setStyle(null);
-  overlayFeatures.push(clone);
-}
-
-new VectorLayer({
-  map: map,
-  source: new VectorSource({
-    features: overlayFeatures
-  }),
-  style: new Style({
-    image: icons[iconCount - 1]
-  })
-});
-
-map.on('click', function(evt) {
-  const info = document.getElementById('info');
-  info.innerHTML =
-      'Hold on a second, while I catch those butterflies for you ...';
-
-  window.setTimeout(function() {
-    const features = [];
-    map.forEachFeatureAtPixel(evt.pixel, function(feature) {
-      features.push(feature);
-      return false;
-    });
-
-    if (features.length === 1) {
-      info.innerHTML = 'Got one butterfly';
-    } else if (features.length > 1) {
-      info.innerHTML = 'Got ' + features.length + ' butterflies';
-    } else {
-      info.innerHTML = 'Couldn\'t catch a single butterfly';
-    }
-  }, 1);
-});
-
-map.on('pointermove', function(evt) {
-  if (evt.dragging) {
-    return;
-  }
-  const pixel = map.getEventPixel(evt.originalEvent);
-  const hit = map.hasFeatureAtPixel(pixel);
-  map.getTarget().style.cursor = hit ? 'pointer' : '';
-});