observing monster become invisible

Requested by one of the beta testers 13 months ago... when a visible
monster becomes invisible and vanishes, mark its map location with
the remembered, unseen monster glyph.  (When the player zaps a
monster with a wand of make invisible, that only happens if the wand
type is known.  I'm not sure that's right but didn't alter it....)

The request suggested also doing it for a monster who disappears by
teleporting away, but I haven't attempted to implement that.

src/mcastu.c

@@ -443,6 +443,8 @@ int spellnum;
                 pline("%s suddenly %s!", Monnam(mtmp),
                       !See_invisible ? "disappears" : "becomes transparent");
             mon_set_minvis(mtmp);
+            if (cansee(mtmp->mx, mtmp->my) && !canspotmon(mtmp))
+                map_invisible(mtmp->mx, mtmp->my);
             dmg = 0;
         } else
             impossible("no reason for monster to cast disappear spell?");