Bump to resvg 0.37 + fontdb 0.16

[LaurenzV]

A couple of the internals of resvg have been changed, so some parts of the codes had to be changed (most notably that text nodes will not directly be replaced by path nodes when converting but instead will be preseved and have a "flattened" attribute).

I also made the cli the default member so that running cargo build from the root directory just works.

And let's maybe first merge #45 before this so they don't have to rebase again.

[laurmaedje]

45 wasn't rebased yet anyway, so I think this is fine to merge.

[laurmaedje]

Thanks!