Don't allow the LoopbackPort to "clone" a URL

Note that `URL`s aren't supported by the structured clone algorithm, see https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm, and any attempt to send a `URL` using `postMessage` is rejected by the browser. Hence, for consistency when workers are disabled, the `LoopbackPort` should obviously also reject any `URL`s.

src/display/api.js

@@ -1800,7 +1800,10 @@ class LoopbackPort {
         }
         return result;
       }
-      result = Array.isArray(value) ? [] : {};
+      if (value instanceof URL) {
+        throw new Error(`LoopbackPort.postMessage - cannot clone: ${value}`);
+      }
+      result = Array.isArray(value) ? [] : Object.create(null);
       cloned.set(value, result); // Adding to cache now for cyclic references.
       // Cloning all value and object properties, however ignoring properties
       // defined via getter.