Lists the pages a viewport shows, in order, and the pixels of each that show. A paged mode shows only the spread at the viewport's centre.
public int GetVisiblePages(in Viewport viewport, Span<VisiblePage> destination, double margin = 0)
The viewport.
Receives the pages, as many as fit.
Pixels added on every side of the viewport, for render-ahead.
destination holds, call again with more room.