Starts the render worker once (later calls share the first): it boots the app's runtime in a Web Worker and waits until the app's
Main has called Serve(Func<PdfEngine>).
public static Task<bool> StartAsync(TimeSpan? timeout = default(TimeSpan? ))
How long to wait for the worker, or null for 60 seconds.
true when tiles can be rendered in the worker; otherwise the viewer draws them itself.