use side_t where appropriate
add a shared lock to map.planes, and hide the original vector to prevent non-locked read/writes
move MakeSkipTexinfo to the beginning of the operation - make a global skip texinfo as the first texinfo. it will get culled out later if need be.
re-enable ChooseMidPlaneFromList; seems to work, but maybe there's a test that is failing that is the reason it was if'd out?
enable parallelization of PartitionBrushes since map.planes is lockable now
8f50108b5e