Builds page's display list and starts drawing it into the top-left GetPixelSize(PdfPage)
pixels of target, on white, in time slices: each Continue(TimeSpan) draws until its
budget is used.
public ProgressiveRender BeginRender(PdfPage page, Bitmap target, RenderOptions options)
The page.
The bitmap, at least the page's pixel size; pixels outside the page's area are left as they are.
The scale and rotation, the cancellation token and the progress receiver.
Viewer capability 2 (issue #97). The display list is built here, at once (cancellable, reporting progress from 0 to 0.5); the slices draw it, reporting from 0.5 to 1. To keep a long build off a thread that must not block, build the list elsewhere (CreateDisplayList(ContentOptions)) and use BeginRender(DisplayList, Bitmap, in Matrix, CancellationToken, IProgress<Double>?).