bkglyph variable gets initialized to GLYPH_UNEXPLORED so ends
up being returned by get_bk_glyph() if something more interesting
wasn't chosen in the switch statement.
The Windows win32 interface will then use the tile mapped to
GLYPH_UNEXPLORED as a background. The tile is 16x16 all black
pixels. That looked very odd.
Treat GLYPH_UNEXPLORED as an out-of-range value.
Closes#929
5ac96e9318