Prepare IIIFInfo for JSON-LD context extensions

IIIF Image API 3 allows context extensions that should be added to the
info.json's @context property. Therefore, '@context' might be an array
instead of a string.

src/ol/format/IIIFInfo.js

@@ -242,8 +242,12 @@ class IIIFInfo {
     if (this.imageInfo === undefined) {
       return;
     }
-    const context = this.imageInfo['@context'] || undefined;
+    let context = this.imageInfo['@context'] || 'ol-no-context';
-    switch (context) {
+    if (typeof context == 'string') {
+      context = [context];
+    }
+    for (let i = 0; i < context.length; i++) {
+      switch (context[i]) {
         case 'http://library.stanford.edu/iiif/image-api/1.1/context.json':
         case 'http://iiif.io/api/image/1/context.json':
           return Versions.VERSION1;
@@ -251,7 +255,7 @@ class IIIFInfo {
           return Versions.VERSION2;
         case 'http://iiif.io/api/image/3/context.json':
           return Versions.VERSION3;
-      case undefined:
+        case 'ol-no-context':
           // Image API 1.0 has no '@context'
           if (this.getComplianceLevelEntryFromProfile(Versions.VERSION1) && this.imageInfo.identifier) {
             return Versions.VERSION1;
@@ -259,6 +263,7 @@ class IIIFInfo {
           break;
         default:
       }
+    }
     assert(false, 61);
   }