Optimize get_heat_radiation

Cache a variable, use the already fetched maptile to check the terrain
at the tile instead of looking up the submap again.

src/game.cpp

@@ -1273,16 +1273,18 @@ int get_heat_radiation( const tripoint &location, bool direct )
     int best_fire = 0;
     Character &player_character = get_player_character();
     map &here = get_map();
+    // Convert it to an int id once, instead of 139 times per turn
+    const field_type_id fd_fire_int = fd_fire.id();
     for( const tripoint &dest : here.points_in_radius( location, 6 ) ) {
         int heat_intensity = 0;
 
         maptile mt = here.maptile_at( dest );
 
-        int ffire = maptile_field_intensity( mt, fd_fire );
+        int ffire = maptile_field_intensity( mt, fd_fire_int );
         if( ffire > 0 ) {
             heat_intensity = ffire;
         } else  {
-            heat_intensity = here.ter( dest )->heat_radiation;
+            heat_intensity = mt.get_ter()->heat_radiation;
         }
         if( heat_intensity == 0 ) {
             // No heat source here