Drop map_shadow, use a bitmap and update it incrementally.

Copying the whole map_knowledge every turn was really costly.  We don't care
about any fields of the map other than the "seen" flag, too.  Thus, we can
move it into a bitmap and update only changes rather than everything.

This means we need to update the entire bitmap once exploration starts, but
that's a small fraction of the cost:

qw profiles:
before:
9.55%     check_for_interesting_features()
0.18%     start_explore(bool)
after:
0.29%     start_explore(bool)
0.10%     check_for_interesting_features()

1 files changed, 2 insertions, 4 deletions

diff --git a/crawl-ref/source/stairs.cc b/crawl-ref/source/stairs.cc
index de9c82da02..17baab785b 100644
--- a/crawl-ref/source/stairs.cc
+++ b/crawl-ref/source/stairs.cc
@@ -464,9 +464,8 @@ void up_stairs(dungeon_feature_type force_stair)
 
     _update_travel_cache(old_level, stair_pos);
 
-    env.map_shadow = env.map_knowledge;
     // Preventing obvious finding of stairs at your position.
-    env.map_shadow(you.pos()).flags |= MAP_SEEN_FLAG;
+    env.map_seen.set(you.pos());
 
     viewwindow();
 
@@ -1051,9 +1050,8 @@ void down_stairs(dungeon_feature_type force_stair, bool force_known_shaft)
     trackers_init_new_level(true);
     _update_travel_cache(old_level, stair_pos);
 
-    env.map_shadow = env.map_knowledge;
     // Preventing obvious finding of stairs at your position.
-    env.map_shadow(you.pos()).flags |= MAP_SEEN_FLAG;
+    env.map_seen.set(you.pos());
 
     viewwindow();