Rework clip paths internal representation #154
Description
The different supported formats represent clip paths differently.
SVG: Clip paths have linking to affected paths in various ways
VD: Paths after the clip path in the same group are affected
IV: Clip paths are specified on groups. The affected paths are children of the group.
Ideally the internal representation represents this in a way that can robustly represent all of the supported formats. Right now the clip path class is a ContainerElement, but it is only really a hack for the SVG case, which is container-like. Because the current structure doesn't robustly associate clip paths with affected paths, converting between the formats can be tricky and adopting a new format (like the recent work to support IV) can add even more complexity.