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, 0 deletions

diff --git a/crawl-ref/source/viewmap.cc b/crawl-ref/source/viewmap.cc
index a46716e911..bb8e0e760b 100644
--- a/crawl-ref/source/viewmap.cc
+++ b/crawl-ref/source/viewmap.cc
@@ -716,6 +716,7 @@ static void _unforget_map()
             // Don't overwrite known squares, nor magic-mapped with
             // magic-mapped data -- what was forgotten is less up to date.
             env.map_knowledge(*ri) = old(*ri);
+            env.map_seen.set(*ri);
         }
 }
 
@@ -727,6 +728,7 @@ static void _forget_map()
             continue;
         env.map_knowledge(*ri).flags &= ~MAP_SEEN_FLAG;
         env.map_knowledge(*ri).flags |= MAP_MAGIC_MAPPED_FLAG;
+        env.map_seen.set(*ri, false);
     }
 }