corridor engravings in black and white

The default engraving-in-corridor character is the same as the default
corridor symbol (and also default lit corridor one), distinguished by
color.  Show it differently (in inverse vidoe, like lava vs water and
sink vs fountain) if color is Off.

It might be better to change the engraving-in-room symbol to be the
same as the room one so that they'll be more consistent with corridors;
color is probably sufficient without resorting to back-tick.  But this
update hasn't done that.

include/display.h

@@ -1026,6 +1026,7 @@ enum glyph_offsets {
 #define MG_BW_ICE  0x00200  /* similar for ice vs floor */
 #define MG_BW_SINK 0x00200  /* identical for sink vs fountain [note: someday
                              * this may become a distinct flag */
+#define MG_BW_ENGR 0x00200  /* likewise for corridor engravings */
 #define MG_NOTHING 0x00400  /* char represents GLYPH_NOTHING */
 #define MG_UNEXPL  0x00800  /* char represents GLYPH_UNEXPLORED */
 #define MG_MALE    0x01000  /* represents a male mon or statue of one */