A bounding box to limit scope of the BSP tree calculations, splitting and merging

[hrgdavor]

I have an idea that there are cases where I know only small part of the model is affected by booelans.
I could provide a bounding box to limit scope of the generic boolean operation and improve performance by reducing the number of polygons checked.

All of the boolean operations would work as right now inside the bounding box, outside of the bounding box:
- subtract would keep "normal" shapes, but discard inverted parts
- intersection would discard all
- union would keep all

Maybe even more than one bounding box would be beneficial ..maybe.

One question is if having this built-in the boolean engine would bring more performance than doing the bounding box shape cutting outside of the boolean engine.