Support drawing svg subsets

[ruuda]

From Reddit: it would be nice to add the ability to render subsets of an svg file.

Rsvg does support this with rsvg_render_handle_cairo_sub and rsvg_handle_has_sub, it would not be very difficult to expose this.

What would be required to implement this:

• Add the ffi wrappers for rsvg_render_handle_cairo_sub and rsvg_handle_has_sub in src/rsvg.rs.
• Change elements::Element::Svg to include an optional svg element id, and maybe define a wrapper struct so it can be named.
• Add a variant of builtins::image that supports subset selection, perhaps image_subset(fname: string, element_id: string).
• Expose the new function in runtime::Env::new. (Handling of builtins needs some refactoring, but for now that is where builtins are exposed.)