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 ...

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();
 }
 
 /*