Bigrm-4 fix and variation

The "butterfly" shaped bigroom was missing the four fountains
in the corners of the room - this was due to my copy-paste error
when converting the bigrooms from des to lua.

Also add some variation to the bigroom.

doc/fixes3-7-0.txt

@@ -976,6 +976,7 @@ wielding a bec de corbin makes ravens generate peaceful
 moving with 'm' prefix allows hero to enter a known pit carefully
 rangers always succeed in disarming bear traps, unless impaired
 bigroom variant 2 may have ice floor in unlit areas
+bigroom variant 4 may have two large squares of terrain in the middle
 some large monsters can knock back smaller monsters with a hit
 change Demonbane to a mace, make it the first sac gift for priests,
 	and give it an invoke ability to banish demons