When moat and lava use the same screen symbol and color is Off, lava
is rendered in inverse video. It used to be similar for floor and
ice, but that got broken last year. Fix inverse ice, and now that
fountain and sink might be same symbol (recent IBMgraphics change),
render sinks in inverse if they match fountains and color is Off.
I started to give sink its own mapglyph flag but then got lazy and
used the same value as ice. That can be amended if some interface
wants to use some more elaborate distinction than inverse video.
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