Fix first person rendering of columns with every cube filled

[AleWin32]

I've been doing some experiments with maps in KeeperFX and found a bug.

I created a modified version of the Eversmile map featuring a large pyramid to showcase all possible height levels.

Eversmile with a big room with a piramid (all is well rendered):

Now with possession mode (bug, the topmost level of the pyramid has a wall extending all the way up to the ceiling):

Walking in possession mode on the topmost level of the pyramid shows no floor:

Fixed:

The ceiling vertex and the top corner of a fully-filled column (all
COLUMN_STACK_HEIGHT cubes) shared the same slot in struct EngineCol. The
ceiling gets written after the column, so it overwrites that corner, and the
column's top face and topmost side stretch up to ceiling height instead of
its own.

Never showed up before because every fully-filled column in the stock maps is
blocking, and a blocking column's own height is also its ceiling, so the two
values always matched by coincidence. Shows up on a walkable one, whose
ceiling comes from the surrounding walls instead.

Isometric and cluedo view are unaffected, they don't draw a ceiling.

Gives the ceiling its own slot past the cube and abyss-wall ones.