Refactor filter code

More and more moving things around, giving names to types, etc. with the goal of having drawing_ctx construct the user_space_filter and user_space_primitives itself, and pass them down to the filters engine.

This MR doesn't quite reach that point, but almost! filters::render() does the element-walking mess in a separate function, extract_filter_from_filter_node, which creates a FilterSpec. Then it just iterates the UserSpacePrimitives inside that and renders them.