Canvases on inactive PagePanel pages receive unnecessary Paint events

Canvases on inactive PagePanel pages can still receive automatic or deferred Paint events and perform drawing work while their page is hidden. Unnecessary paints should be deferred until the Canvas becomes visible, with repeated requests combined into a repaint using the latest state.

Explicit Refresh() and Refresh(x, y, width, height) must keep their existing behaviour: each call runs Paint synchronously, including when the Canvas is on an inactive page.