Add support for OT-in-SVG for freetype backend

Freetype 2.12 added support for OT-in-SVG by providing hooks to render svgs. It seems like it doesn't force to use librsvg, at least it seems like it. This is required if we want to support certain emoji fonts like `twemoji` or apple emoji fonts.

https://freetype.org/freetype2/docs/reference/ft2-ot_svg_driver.html