When a monster is drawn on the map, remove any "remembered, unseen
monster" glyph being shown at the same spot. Clairvoyance shows
all monsters in vicinty, then ones which can't be seen are replaced
with the 'I' glyph (which is on the object layer or the display,
not the monster layer show is subject to different update behavior).
But subsequent monster refresh didn't get rid of it when a sensed
monster was displayed over it. (3.6.1 included a similar fix for
warned-of monsters.)
Also during clairvoyance, don't draw an 'I' at a spot that will
immediately be refreshed with a monster because 'I' clobbers any
remembered object at the same location.
0c98a30b8d