Add an example of clipping layer based on a vector source

examples/layer-clipping-vector.css

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+#map {
+  background: transparent;
+}

examples/layer-clipping-vector.html

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+---
+layout: example.html
+title: Vector Clipping Layer
+shortdesc: Vector Clipping Layer example
+docs: >
+  Example of a clipping layer based on a vector source
+tags: "clipping, openstreetmap, vector"
+---
+<div id="map" class="map"></div>

examples/layer-clipping-vector.js

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+import Map from '../src/ol/Map.js';
+import View from '../src/ol/View.js';
+import {Tile as TileLayer, Vector as VectorLayer} from '../src/ol/layer.js';
+import VectorSource from '../src/ol/source/Vector.js';
+import GeoJSON from '../src/ol/format/GeoJSON.js';
+import OSM from '../src/ol/source/OSM.js';
+import {Fill, Style} from '../src/ol/style.js';
+import {getVectorContext} from '../src/ol/render.js';
+import {fromLonLat} from '../src/ol/proj.js';
+
+const base = new TileLayer({
+  source: new OSM()
+});
+
+const clipLayer = new VectorLayer({
+  style: null,
+  source: new VectorSource({
+    url:
+    './data/geojson/switzerland.geojson',
+    format: new GeoJSON()
+  })
+});
+
+const style = new Style({
+  fill: new Fill({
+    color: 'black'
+  })
+});
+
+base.on('postrender', function(e) {
+  e.context.globalCompositeOperation = 'destination-in';
+  const vectorContext = getVectorContext(e);
+  clipLayer.getSource().forEachFeature(function(feature) {
+    vectorContext.drawFeature(feature, style);
+  });
+  e.context.globalCompositeOperation = 'source-over';
+});
+
+const map = new Map({
+  layers: [base, clipLayer],
+  target: 'map',
+  view: new View({
+    center: fromLonLat([8.23, 46.86]),
+    zoom: 7
+  })
+});