Refactoring the display class quickly didn't work.

So now we'll do it the slow step-by-step way, starting by subclassing
display from an empty superclass and moving a little bit of stuff up
before each test.

src/display.cpp

@@ -48,6 +48,15 @@
 #define ERR_DP LOG_STREAM(err, display)
 #define INFO_DP LOG_STREAM(info, display)
 
+
+map_display::map_display()
+{
+}
+
+map_display::~map_display()
+{
+}
+
 std::map<gamemap::location,fixed_t> display::debugHighlights_;
 
 namespace {

src/display.hpp

@@ -50,7 +50,14 @@ class unit_map;
 //   currently moused over (highlighted)
 // - the unit status, which displays an image for, and stats for, the
 //   current unit.
-class display
+class map_display
+{
+public:
+	map_display();
+	~map_display();
+};
+
+class display : public map_display
 {
 public:
 	display(unit_map& units, CVideo& video,