Make svg_fmt dependency optional

[hecrj]

Hey, thanks for this nice library!

We are planning to use it in iced_wgpu to generate image atlases: iced-rs/iced#154

I noticed the svg_fmt dependency is only used in the dump_svg function, which seems to satisfy a very specific use case. This PR makes this dependency optional.

[nical]

We are planning to use it in iced_wgpu to generate image atlases: iced-rs/iced#154

Cool!

I noticed the svg_fmt dependency is only used in the dump_svg function, which seems to satisfy a very specific use case. This PR makes this dependency optional.

svg_fmt is a super tiny crate so it won't make a difference in compile times or binary size, but I don't mind putting it behind a flag.

[nical]

Sorry I ended up reverting this change. svg_fmt is tiny and takes less than half a second to build so I changed my mind and decided it wasn't worth the extra feature flag.

[hecrj]

No worries. It just surprised me to see an svg_fmt dependency when running cargo tree, as the crate seems quite generic.