One draw of a frame (GetFrame(in Viewport, Span<FramePart>)): a page's paper, or a tile already on the surface, and where it goes in the viewport.
public FramePart(FramePartKind Kind, Tile Tile, LayoutRectangle Destination)
What it draws.
The tile on the surface to draw (as Present(Tile, Bitmap) received it), stretched from its whole bitmap onto
Destination; for Paper, the page's whole render at the frame's zoom (nothing to fetch).
Where it goes, in viewport pixels; it may reach past the viewport's edges, which the shell clips.
Spec #33 viewer capability 2. Parts come in drawing order (later parts over earlier): every page's paper, then the scaled tiles farthest from the frame's zoom first, then the sharp tiles, so the sharpest pixels end on top.