Add vl-convert test specs and assorted fixes

[jonmmease]

Adds nearly all of the specs from the vl-convert test suite, and includes a bunch of fixes to make them pass:

• Don't leave space for circle stroke when stroke is transparent
• Use a user agent when fetching images with reqwest so that OSM servers are happy
• Handle dx/dy in text marks
• Support remote images in SVG format using resvg
• Allow x/y props to be missing and null across all mark types
• Switch from u16 to u32 indices in the basic mark type to support large paths
• Allow text property of Vega text mark to be a number or null
• Support sorting marks within groups by their zindex

Closes #46, #48