plug-ins: add back wireframe mode using cairo

plug-ins/map-object/map-object-preview.h

@@ -4,6 +4,8 @@
 #define PREVIEW_WIDTH 200
 #define PREVIEW_HEIGHT 200
 
+#define WIRESIZE 16
+
 /* Externally visible variables */
 /* ============================ */
 
@@ -16,6 +18,11 @@ extern gint       lightx,lighty;
 void     compute_preview_image  (void);
 gboolean preview_expose         (GtkWidget      *widget,
                                  GdkEventExpose *eevent);
+void     draw_wireframe         (gint startx,
+                                 gint starty,
+                                 gint pw,
+                                 gint ph);
+void     draw_preview_wireframe (void);
 gint     check_light_hit        (gint            xpos,
                                  gint            ypos);
 void     update_light           (gint            xpos,