Add BSP algorithm for SVGRenderer

[the-owl]

SVGRenderer has some issues with z-sorting: painter sort which is currently used sometimes gives wrong results and it can't deal with intersecting objects. As a solution, I've implemented an algorithm that builds a BSP tree and splits objects into smaller pieces. Here is an example (set useBSP = false to see the difference).
It only supports triangles and lines, but it can probably be improved in future.

[mrdoob]

Very nice! The good thing is that this also works for CanvasRenderer! I guess it doesn't handle UVs though?

[the-owl]

I didn't expect this to work with CanvasRenderer, so I didn't add UVs handling because SVGRenderer doesn't support textures. It also could be improved in future.

[mrdoob]

I see... Do you think you could add UVs to this PR?

[the-owl]

I've added UV handling and an option for CanvasRenderer

[ToJans]

This might be a long shot, but if I understood correctly transparent object rendering might also benefit from this?

[Mugen87]

Not sure this PR is still necessary. Just because SVGRenderer seems a bit deprecated (and CanvasRenderer already removed). In the meanwhile you can render SVG files with WebGL thanks to SVGLoader (see #13616).

Besides, when reading the code I've noticed a lot of objects are created when the BSP tree is computed per frame. This approach might lead to noticeable GC pressure. Without knowing the performance implications, it seems to be hard to merge such a change...

[Mugen87]

Closing for now.