Optimize|BSP Builder: Allocate vertexes for BSP line segments from the map's Mesh

The space partitioner now allocates vertexes from the Mesh owned by
the map directly. This means the map does not have to take ownership
of the vertexes and can index them immediately.

Thusly there is no longer an "editable" set of vertexes as they can
all be considered editable until they are linked to some higher level
geometry construct such as a map line.

Todo for later: Vertexes for "extra" meshes would ideally come from
the same Mesh as their HEdges. This would mean assigning new vertexes
to some map lines (which is already implemented) however ultimately
it doesn't really matter which mesh a given vertex originates from.