* lighting_overlay objects belong to a turf. Turfs never move. Neither should the lighting_overlay, even of an overzealous shuttle controller thinks they should.
The previous client-side optimization made lighting overlays go fully
opaque when it was dark, making it so species darksight didn't let you see
anything. This fixes that by making a dark_soft icon state that has
opacity matching the LIGHTING_SOFT_THRESHOLD.
Removes lighting transitions- they don't work with the new system.
Greyscale overlays now use BLEND_OVERLAY instead of BLEND_MULTIPLY for
better client performance.
Light overlays in total darkness are now made invisible.
Signed-off-by: Mloc <colmohici@gmail.com>