To test 16-color mode, specify OPTIONS=video:vga explicitly;
autodetect will choose a VESA mode if it can.
* Draw tiles correctly when redrawing from panning or from changing
the map mode among text, tiles and overview. Previously, this would
draw everything with the tile at the hero's position.
* Draw corridor walls with the stone tile.
* Map the statue colors to the nearest neutral tone among the main
16 colors. This mainly affects altars and female cats.
* Fix the code that shows statues as the generic statue tile. This
code could be deleted, but the statues don't draw with the full
range of gray tones.
* Document the option OPTIONS=video:vesa.
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