SSpreadsheetZoomViewport class

Renders child inside a logical viewport of incoming size / zoom and paints it at zoom.

This is the primitive SSpreadsheet.zoom is built from, exposed so that any layer sharing the sheet's coordinate space — an overlay positioned from the same row/column dimensions, a HUD pinned to the grid — can be scaled by exactly the same amount and stay aligned with the sheet.

Scaling down enlarges the logical viewport rather than shrinking the content into a corner, so zooming out reveals more rows and columns instead of leaving empty space around a smaller copy of the sheet; scaling up shrinks the logical viewport, so fewer of them fit — which is what zooming in means for a scrollable grid.

Two cases pass child through unwrapped, so they cost nothing and cannot regress an existing layout:

• zoom == 1.0, the default everywhere;
• unbounded incoming constraints, where there is no viewport to divide — the child is then laid out as it would have been, rather than unbounded or with a meaningless logical size.