experimental SVG support

Adds a feature flag "svgomg-enabled" that directs SVG exports at an experimental SVG export engine: https://github.com/adobe-research/svgObjectModelGenerator

Digging into the SVGOMG code, it _looks_ like this is intended to have the same format as the layerSpec in `getPixmap`. Is that correct?

I'm going to add that documentation. If that's wrong, please correct it.