This adds a pair of new glyphs: GLYPH_UNEXPLORED and GLYPH_NOTHING
GLYPH_UNEXPLORED is meant to be the glyph for areas of the map that
haven't been explored yet.
GLYPH_NOTHING is a glyph that represents that which cannot be seen,
for instance the dark part of a room when the dark_room option is
not set. Since the symbol for stone can now be overridden to
a players choice, it no longer made sense using S_stone for the
dark areas of the room with dark_room off. This allows the same
intended result even if S_stone symbol is mapped to something visible.
GLYPH_UNEXPLORED is what areas of the map get initialized to now
instead of STONE.
This adds a pair of new symbols: S_unexplored and S_nothing.
S_nothing is meant to be left as an unseen character (space) in
order to achieve the intended effect on the display.
S_unexplored is the symbol that is mapped to GLYPH_UNEXPLORED, and
is a distinct symbol from S_stone, even if they are set to the same
character. They don't have to be set to the same character.
Hopefully there are minimal bugs, but it is a deviation from a
fairly long-standing approach so there could be some unintended
glitches that will need repair.
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