Ported 'see-down' open spaces from Vore, who ported it from Eris
Ported talking and visible messages traveling upwards through open spaces
Tested, seems to work? Does not seem to break anything, maybe.
Maybe made a changelog.
* Instead of open spaces simply showing a static image of the turf below, it shows the turf, all objects, and even moving mobs.
* Controller updates the view to floor below fairly quickly; mobs update in real time.
* Open space can be examined to see how deep it is.
* Mobs are examinable even when on floor below, using a special "zshadow" mob.
* Changes from Eris: Heavily re-written to handle layering, special cases, rewrote controller etc. Shadow mobs don't break entering turfs, etc.
* Update to properly layer the objects from multiple "below" floors in the case of tripple-stacked or more z levels.
* Added constant defines for the planes in use.
* Added open_space.dmi with blank_open icon state for open space's darkening overlay.