Fix fullscreen on Safari (#814)

* Fix fullscreen on Safari

Safari only supports it via the webkit prefix

Fixes https://github.com/vector-im/element-call/issues/539

* NOT THAT LOGGER, VSCODE

src/@types/global.d.ts

@@ -27,4 +27,9 @@ declare global {
   interface MediaElement extends HTMLVideoElement {
     setSinkId: (id: string) => void;
   }
+
+  interface HTMLElement {
+    // Safari only supports this prefixed, so tell the type system about it
+    webkitRequestFullscreen: () => void;
+  }
 }