Add support for `vw`/`vh` units
See for example the following SVG:
<svg width='100vw' height='100vh' xmlns='http://www.w3.org/2000/svg' xmlns:xlink='http://www.w3.org/1999/xlink'>
<rect width='100%' height='100%' style='fill:#000000;'/>
<rect width='100%' height='100%' rx='50' ry='50' style='fill:#ffffff;'/>
</svg>
Edited by Sebastian Dröge