Support SVG input

[jonmmease]

The primary goal of VlConvert is the conversion of Vega and Vega-Lite charts to other formats. But the PNG and PDF export workflows both work by first converting a chart to an SVG image, then converting the SVG image to either PNG or PDF. Because of this, it would be fairly straightforward to support conversions that input general SVG images: svg_to_png and svg_to_pdf.

This would allow VlConvert to serve a purpose similar to svglib's svg2pdf functionality, but with a permissive license (svglib is LGPLv3 licensed). Or similar to CairoSVG's svg2png/svg2pdf with a permissive license (CairoSVG uses LGPLv3) and without a dependency on a system install of the cairo shared library.