Add support for SVG 2 viewport and root-relative units: rem, vw, vh, vmin, vmax, q

[deeplook]

Summary

svglib does not implement the CSS/SVG 2 length units introduced after SVG 1.1.
These units are increasingly common in modern SVG content.

Missing units

Unit	Meaning	Resolution strategy
rem	Root element font-size	Capture root font-size during render and use as rem base
vw	1% of viewport width	Resolve against main_box.width (already tracked)
vh	1% of viewport height	Resolve against main_box.height (already tracked)
vmin	Smaller of vw and vh	min(vw, vh)
vmax	Larger of vw and vh	max(vw, vh)
q	Quarter-millimetre (0.25 mm)	Straightforward physical conversion

ch is already approximated as 0.5em.

Notes

• vw/vh should treat the outermost SVG canvas as the viewport (the PDF
  render target has no browser viewport).
• rem requires storing the computed root font-size on Svg2RlgAttributeConverter
  before the tree walk begins.
• The convertLength method in Svg2RlgAttributeConverter already has the
  right structure to add these as new elif branches.
• Target: 2.1 (after 2.0 coordinate-system change stabilises).