levels: Improve/complete abandoned MAP10.

More context can be found in this thread:
http://www.doomworld.com/vb/freedoom/69412-updated-map10

This level appears to have been abandoned by its original author and
Jewellds took it upon himself to continue the work.

Changes since v21:
-The lava building is now accessed by a red key, rather than a switch,
 and the small building to the south has changed as a result.
-Minor cosmetic improvements to the lamps by the blue key door.
-Lowered some sectors in the river by the building entrance, to make
 it look more realistic.
-Fixed a crate in the Mancubus room, which was too tall.
-Removed a misplaced midtexture on the cave entrance.
-Fixed a few texture alignment and cosmetic issues surrouding (what is
 now) the red key door.
-Solved some alignment problems on the rocks in the south-east of the
 map.