librsvg: alternatives and conditional compilation?

Current librsvg is used to render OT-SVG glyphs. However, librsvg is far from simple to build as it requires tons of transitive dependencies, especially on Windows.

Therefore, we could introduce a configure switch in the meson script (as well as autoconfs?) to switch librsvg on and off. This is trivial because the source code itself already has a HAVE_LIBRSVG compile switch.

Additionally, it would be great if we could support alternative SVG rendering libraries like Lunasvg and resvg, and allow switching between those libraries in the build script.