Gets or sets what receives a page render's progress, from 0 to 1; null, the default, for none.
public IProgress<double>? Progress { get; set; }
Viewer capability 2 (issue #97). Reported synchronously on the rendering thread: building the display list moves it from 0 to 0.5 (Progress, halved), drawing from 0.5 to 1 (the share of the list's operations drawn, every 1,024 operations and at the end of every slice of a ProgressiveRender). Values never decrease, and the last report of a render that completes is exactly 1, reported once. The receiver may cancel CancellationToken's source.