Re-organise zooming. Do just one rendering and scaling of the scene.

The zoom range (0 to 200) is adjusted to fit each puzzle as it loads,
with close-up view near one end and full-puzzle view near the other and
smoother operation in between. The close-up view is always larger than
the full view and can never be closer than a noticeable margin, even in
puzzles with a small number of pieces.

The scene is populated with items and sized only when all pieces have
been loaded from files, constructed, positioned and joined together (if
previously solved). A frame is added and the whole scene is fitted into
the view closely, showing the whole puzzle. Loading times are improved.