Bound PDF art by containment, not drawing-order windows

When rebuilding one design from a PDF page's vector paths, don't assume the next N drawings belong to the current panel. A 16-drawing window after a tile's background spilled into the neighbouring tile's eyes and produced a wrong union bbox (a 2 x 2 in tile instead of 2 x 1.65 in). Select only drawings whose rect is contained in the panel or background rect. Designer boards can also hold hidden shapes and their child parts, so exclude those too. Check by rendering each extracted file, because the wrong bbox won't raise an error.