SolidObject: introduce STATE_BIT_INVOID

units with this bit set will never
  1) take damage, even from Lua (but can be killed by Lua)
  2) be rendered through any engine path (nor their icons)
  3) be intersectable by either synced or unsynced rays
  4) block any other units from existing on top of them

main use: removing the ancient "move transportee 1000 elmos
under the map to hide it when attaching to piece -1" hack