Partitioner: Added build option DENG_BSP_COLLAPSE_ORPHANED_LEAFS

Enables collapsing of orphaned BSP leafs whilst partitioning the map
geometry and building the BSP.

This feature needs testing. It solves several rendering issues and
many render-hacks, however, presently the back sector is cleared for
any lines which produced an orphan (meaning it may adversely affect
any line specials dependent on said lines).

At this stage I'm only really interested in locating any adverse
side effects the feature may have on geometry construction/rendering.

I'll return to this once I've completed the current work phase.