parse radialGradient radius as percentage of viewport diagonal, not width

[anthrotype]

currently we do:

picosvg/src/picosvg/svg_types.py

Line 842 in 9d00851

self.r = number_or_percentage(el.attrib.get("r", "50%"), width)

But the SVG spec says

For any other length value expressed as a percentage of the SVG viewport, the percentage must be calculated as a percentage of the normalized diagonal of the ‘viewBox’ applied to that viewport. If no ‘viewBox’ is specified, then the normalized diagonal of the SVG viewport must be used. The normalized diagonal length must be calculated with sqrt((width)**2 + (height)**2)/sqrt(2).

So we should do the right thing (even if percentages usually are used with objectBoundingBox where the unit square by definition is width==height==1).

I push the failing test first, then the fix