Allow returning the source of edges from BoolOp.

[andriyDev]

BoolOps take two (multi-)polygons, the subject and the clip and returns the union/intersection/XOR/difference of these as a new (multi-)polygon. It can be useful to know which edges in the result polygon come from which edges in the subject and the clip.

Example:
I have room A with wallpaper A, and a hallway C with wallpaper C. I want to attach these, so I take the union between room A's polygon and hallway C's polygon. Now what wallpaper do I assign to each wall in the union polygon?

Based on my understanding of the Martinez-Rueda-Feito algorithm:

• When we create events initially, we can just attach some metadata saying which polygon (subject or clip) and which edge in that polygon the event polygons to.
• Splitting an edge does not change this metadata. We can just copy it over from the existing edge.
• Any overlapping edges that are detected need to store the metadata of both edges in them (so whichever edge is in the result will have the full information).
• When linking up the result polygon, we just also store the metadata alongside it.

This does mean we require a little more memory per event, and returning this metadata will require an extra allocation. Perhaps we can avoid the extra memory allocation by adding a new trait: BoolOpsWithEdgeSource?