Lazy evaluation of overview info

Callgrind showed recalc_mapseen was three times more expensive (in terms
of instructions read) than anything else in our codebase.  It was being
called in every vision change, re-evaluating the last seen map terrain
type for every map location in sight.

Remove updating the lastseen info in the vision code, and make a small
change so newsym() uses update_lastseentyp.

From my short tests, this seems to work correctly ...

include/extern.h

@@ -828,6 +828,7 @@ extern int dooverview(void);
 extern void show_overview(int, int);
 extern void rm_mapseen(int);
 extern void init_mapseen(d_level *) NONNULLARG1;
+extern void update_lastseentyp(coordxy, coordxy);
 extern void recalc_mapseen(void);
 extern void mapseen_temple(struct monst *);
 extern void room_discovered(int);

src/display.c

@@ -152,7 +152,6 @@ static void set_seenv(struct rm *, coordxy, coordxy, coordxy, coordxy);
 static void t_warn(struct rm *);
 static int wall_angle(struct rm *);
 
-#define remember_topology(x, y) (gl.lastseentyp[x][y] = levl[x][y].typ)
 #define _glyph_at(x, y) gg.gbuf[y][x].glyphinfo.glyph
 
 /*
@@ -256,7 +255,7 @@ magic_map_background(coordxy x, coordxy y, int show)
     if (show)
         show_glyph(x, y, glyph);
 
-    remember_topology(x, y);
+    update_lastseentyp(x, y);
 }
 
 /*
@@ -461,7 +460,7 @@ unmap_object(register coordxy x, register coordxy y)
         else                                                                \
             map_background(x, y, show);                                     \
                                                                             \
-        remember_topology(x, y);                                            \
+        update_lastseentyp(x, y);                                           \
     }
 
 void
@@ -3670,7 +3669,6 @@ fn_cmap_to_glyph(int cmap)
 /* for 'onefile' processing where end of this file isn't necessarily the
    end of the source code seen by the compiler (there are lots of other
    macros defined above...) */
-#undef remember_topology
 #undef _glyph_at
 #undef DETECTED
 #undef PHYSICALLY_SEEN

src/dungeon.c

@@ -64,7 +64,6 @@ static mapseen *find_mapseen(d_level *);
 static mapseen *find_mapseen_by_str(const char *);
 static void print_mapseen(winid, mapseen *, int, int, boolean);
 static boolean interest_mapseen(mapseen *);
-static void update_lastseentyp(coordxy, coordxy);
 static void count_feat_lastseentyp(mapseen *, coordxy, coordxy);
 static void traverse_mapseenchn(int, winid, int, int, int *);
 static const char *seen_string(xint16, const char *);
@@ -3062,7 +3061,7 @@ interest_mapseen(mapseen *mptr)
 }
 
 /* update the lastseentyp at x,y */
-static void
+void
 update_lastseentyp(coordxy x, coordxy y)
 {
     struct monst *mtmp;
@@ -3312,10 +3311,11 @@ recalc_mapseen(void)
      * the ability to have non-dungeon glyphs float above the last known
      * dungeon glyph (i.e. items on fountains).
      */
+    if (!Levitation)
+        update_lastseentyp(u.ux, u.uy);
+
     for (x = 1; x < COLNO; x++) {
         for (y = 0; y < ROWNO; y++) {
-            if (cansee(x, y) || (u_at(x, y) && !Levitation))
-                update_lastseentyp(x, y);
             count_feat_lastseentyp(mptr, x, y);
         }
     }

src/vision.c

@@ -838,8 +838,6 @@ vision_recalc(int control)
     /* Set the new min and max pointers. */
     gv.viz_rmin = next_rmin;
     gv.viz_rmax = next_rmax;
-
-    recalc_mapseen();
 }
 
 /*