A relatively recent change to make secret doors within horizontal walls
become horizontal doors after discovery was making some secret doors
that should have remained vertical become horizontal too. While still
hidden, they got displayed as horizontal wall segments in the midst
of vertical walls. Example was the "Catacombs" (minend-3) variant of
mines' end. The hidden door on the east wall of the entry room was
shown as horizontal, while another one on the west wall of that same
room was correctly vertical. This fix uses different criteria to
decide horizontal vs vertical, partly because I couldn't understand
how the previous code was supposed to work.
Hidden doors now seem to display as correctly oriented walls and once
discovered seem to become correctly oriented doors. I tested by
checking quite a few special levels (and some regular ones)--but not
all--with '#terrain d'. Plus some searching to unhide secret doors
while using a custom symbol set that displayed closed horizontal doors
(S_hcdoor) as '=' and vertical ones (S_vcdoor) as '"'.
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