Percentage lengths might be wrong

[ygra]

Currently we're evaluating those against the root SVG element, which is unlikely to be correct within symbols, markers, and other things that are referenced and instantiated elsewhere. Perhaps we should use the UseContext there as well.

And perhaps we could get rid of the units package in the process, as percentages are pretty much the only thing that's complicated and not just a simple multiplier of CSS pixels. And if we have to do those on our own anyway ...