Fix viewBox dimensions in SVG creation

[noreun]

Fix SVG viewBox using max coordinates instead of width/height

The viewBox attribute in render_to_svg is set to:

"viewBox": f"{pbot[0]} {pbot[1]} {ptop[0]} {ptop[1]}"

SVG viewBox semantics are min-x min-y width height, not min-x min-y max-x max-y. This causes the rendered SVG to be clipped whenever the design's bounding box doesn't start at the origin.

For example, a design spanning [-200, 200] produces viewBox="-200 0 200 10000" (a 200-unit-wide view starting at -200, clipping everything past x=0).

Fix:

"viewBox": f"{pbot[0]} {pbot[1]} {width_points} {length_points}"

(width_points and length_points are already correctly computed)