Cache the absolute position of the top-left map square

In the case of a character waiting in the refugee centre, calls are in
the order of 10,000,000 each in-game hour.

src/map.cpp

@@ -10045,17 +10045,18 @@ tripoint_abs_ms map::getglobal( const tripoint &p ) const
 
 tripoint_abs_ms map::getglobal( const tripoint_bub_ms &p ) const
 {
-    return project_to<coords::ms>( abs_sub.xy() ) + p.raw();
+    return abs_ms + p.raw();
 }
 
 tripoint_bub_ms map::bub_from_abs( const tripoint_abs_ms &p ) const
 {
-    return tripoint_bub_ms() + ( p - project_to<coords::ms>( abs_sub.xy() ) );
+    return tripoint_bub_ms { p.x() - abs_ms.x(), p.y() - abs_ms.y(), p.z()};
 }
 
 void map::set_abs_sub( const tripoint_abs_sm &p )
 {
     abs_sub = p;
+    abs_ms = tripoint_abs_ms{ project_to<coords::ms>(abs_sub.xy()), abs_sub.z()};
 }
 
 tripoint_abs_sm map::get_abs_sub() const

src/map.h

@@ -2251,6 +2251,8 @@ class map
          * - shifting the map with @ref shift
          */
         tripoint_abs_sm abs_sub;
+        // Cached value of tripoint_abs_ms { project_to<coords::ms>( abs_sub.xy() ), abs_sub.z() }
+        tripoint_abs_ms abs_ms;
         /**
          * Sets @ref abs_sub, see there. Uses the same coordinate system as @ref abs_sub.
          */