MAX_WORLD_COORD*2 as radius rather than MAX_WORLD_COORD, this fixes
problems with certain invalid brushes not being rejected because of
insufficiently large coords, usually causing a stack overflow when
constructing the BSP tree (due to insufficient "_blocksize")
git-svn-id: svn://svn.icculus.org/gtkradiant/GtkRadiant/trunk@156 8a3a26a2-13c4-0310-b231-cf6edde360e5