Stroke alignment on «Solidify Stroke»

The «Solidify Stroke» assumes that all strokes are `StrokeAlign::Center` [sic] causing it to give the incorrect results for `StrokeAlign::Inside` and `StrokeAlign::Outside`.

The alignment can be set in the properties of the stroke node:

A proper implementation of the algorithm must deal with intersecting bézier paths in the same way as the SVG and vello renderer. An interesting case can be seen here where we have a closed path made of just two intersecting cubic béziers:

If the path is not fully closed, then the centre alignment must be used to match the renderer. Also note that a path can contain multiple contours. These can likely be dealt with individually?

To deal with intersections of paths in an efficient the [`linesweeper` crate](https://docs.rs/linesweeper) can be used since it is already included as a dependency.

The `kurbo::stroke` function only supports centre alignment. I'm not sure if it is within scope to add other alignments to it.

As reported by [@Menxs on discord](https://discord.com/channels/731730685944922173/1134981447539761273/1544038003955990538).

Start by locating the Solidify Stroke implementation and its StrokeAlign handling. Compare its behavior for open, closed, intersecting, and multi-contour paths with the SVG and vello renderer, and review the available linesweeper dependency alongside kurbo::stroke. Done means Inside, Outside, and Center alignments produce matching results, with open paths retaining center alignment.

Vom Indexierungsmodell aus dem Issue-Text verfasst.