Add IIIF Image API tilesource with example

src/ol/source/IIIF.js contains a tile source for IIIF Image API services.
It supports Image API version 1 and 2 on compliance levels 0, 1 and 2.

To get working constructor options for IIIF from an IIIF image info.json,
use src/ol/format/IIIFInfo.js.

An example is available in examples/iiif.html respectivly examples/iiif.js.

examples/iiif.html

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+---
+layout: example.html
+title: IIIF Image API
+shortdesc: Example of a IIIF Image API source.
+docs: >
+  Example of a tile source for an International Image Interoperability Framework (IIIF) Image Service.
+  Try any Image API version 1 or 2 service.
+tags: "IIIF, IIIF Image API, tile source"
+---
+<div id="map" class="map"></div>
+<div class="controls">
+  <div id="iiif-notification">&nbsp;</div>
+  Enter <code>info.json</code> URL:
+  <input type="text" id="imageInfoUrl" value="https://iiif.ub.uni-leipzig.de/iiif/j2k/0000/0102/0000010218/00000061.jpx/info.json">
+  <button id="display">Display image</button>
+</div>

examples/iiif.js

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+import Map from '../src/ol/Map.js';
+import View from '../src/ol/View.js';
+import TileLayer from '../src/ol/layer/Tile.js';
+import IIIF from '../src/ol/source/IIIF.js';
+import IIIFInfo from '../src/ol/format/IIIFInfo.js';
+
+const layer = new TileLayer(),
+    map = new Map({
+      layers: [layer],
+      target: 'map'
+    });
+
+const notifyDiv = document.getElementById('iiif-notification');
+
+function refreshMap(imageInfoUrl) {
+  fetch(imageInfoUrl).then(function(response) {
+    response.json().then(function(imageInfo) {
+      const options = new IIIFInfo().readFromJson(imageInfo);
+      if (options === undefined || options.version === undefined) {
+        notifyDiv.textContent = 'Data seems to be no valid IIIF image information.';
+        return;
+      }
+      const extent = [0, -options.size[1], options.size[0], 0];
+      layer.setSource(new IIIF(options));
+      map.setView(new View({
+        resolutions: layer.getSource().getTileGrid().getResolutions(),
+        extent: extent,
+        constrainOnlyCenter: true
+      }));
+      map.getView().fit(extent);
+      notifyDiv.textContent = '';
+    }).catch(function(body) {
+      notifyDiv.textContent = 'Could not read image info json. ' + body;
+    });
+  }).catch(function() {
+    notifyDiv.textContent = 'Could not read data from URL.';
+  });
+}
+
+const urlInput = document.getElementById('imageInfoUrl');
+const displayButton = document.getElementById('display');
+
+displayButton.addEventListener('click', function() {
+  const imageInfoUrl = urlInput.value;
+  refreshMap(imageInfoUrl);
+});
+
+refreshMap(urlInput.value);