BSP Builder: Allocate geometry components from the Map's primary Mesh

All components of the primary mesh geometry for the map are now all
allocated from one Mesh instance owned by the map. This simplifies
the map destruction process as BspLeaf is no longer responsible for
free'ing a subsection of the primary geometry.

Note that the geometries of "extra meshes" (i.e., polyobjs and any
special non-contiguous BSP leaf constructs) are unaffected (still
allocated from independent Mesh instances owned by the map element).