Does one display frame's work: advances a fling or an animation to the clock's time, then the zoom's frame
(RunFrame(TimeSpan)), which settles zoom steps and draws tiles within budget.
public bool RunFrame(TimeSpan budget)
The time the frame may spend drawing tiles.
true when the shell should run another frame: something animates, or tiles remain to draw.Spec #33 viewer capabilities 2 and 14. Allocates nothing beyond the tiles the scheduler starts.