`graphics/librsvg2` is stuck on the 2.40 branch due to newer versions requiring a Rust toolchain which we do not have on Tier 2 archs. I packaged a newer librsvg2 version as `graphics/librsvg2-rust`. LIBRSVG2_DEFAULT allows adventurous users to opt into using `graphics/librsvg2-rust` instead of `graphics/librsvg2`.