Lists what a shell draws for viewport from what is on the surface now, before any render for it completes:
each visible page's paper, then every tile on the surface that shows in the viewport, stretched or shrunk from its own scale to
the viewport's zoom, the sharpest last. A zoom step shows at once this way (scaled tiles, never a blank page) while sharp tiles
render at the settled scale.
public int GetFrame(in Viewport viewport, Span<FramePart> destination)
The viewport, which may differ from the one given to Update(in Viewport) (a zoom in progress).
Receives the parts in drawing order.
Spec #33 viewer capability 2. Tiles drawn at another layout rotation are left out. Allocates nothing once its page buffer has grown to the viewport.