Support SVG in static builds

Split from issue 88. See comment by @crmabs there.

I had to leave out SVG support from the static exes because it was hard to support in a static build. This has probably gotten easier in the past year or two.

We depend on a post-Rust-rewrite librsvg2. MXE still has the last C-only version, probably because they can't support a Rust toolchain either. I could probably add an option to use an older API, but old librsvg2 versions don't receive updates, so are probably insecure.