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, 3 insertions, 0 deletions

diff --git a/crawl-ref/source/view.cc b/crawl-ref/source/view.cc
index 751c4df3f1..bb5fb3997d 100644
--- a/crawl-ref/source/view.cc
+++ b/crawl-ref/source/view.cc
@@ -452,7 +452,10 @@ bool magic_mapping(int map_radius, int proportion, bool suppress_msg,
         }
 
         if (env.map_knowledge(*ri).changed())
+        {
             env.map_knowledge(*ri).clear();
+            env.map_seen.set(*ri, false);
+        }
 
         if (!wizard_map && (env.map_knowledge(*ri).seen() || env.map_knowledge(*ri).mapped()))
             continue;