A theoretical implementation of bsp_clip() and bsp_clip_polygons()

Never even ran it, but it sure compiles.
I was iterating the wrong list of polygons.

Sadly, I was not -Werroring on unused variables. My life means nothing.
stl_to_bsp() helper.
Remove the kl_destroy() calls in clip
Attempt to perform a BSP intersection
Actually update the list of node polygons
Delete only when not null
Commit a leak note
Considerations around split polygons being an array, not individuals and a new free_poly()

The new free poly takes a second parameter to ask to free itself instead of it's slots.
This is used to cleanly deallocate split polygons
Merge pull request #11 from sshirokov/clip_to

bsp_clip() and friends