Resolve ""Show overview" behaves incorrectly when unused images in "images" folder"

Closes #1308 (closed)

converter/src/main/java/lcsb/mapviewer/converter/OverviewParser.java

 package lcsb.mapviewer.converter;
 
-import java.awt.*;
+import java.awt.Polygon;
 import java.awt.geom.Area;
 import java.awt.geom.Point2D;
 import java.awt.image.BufferedImage;
 import java.io.*;
 import java.util.*;
-import java.util.List;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipFile;
 
@@ -350,6 +349,30 @@ public class OverviewParser {
       }
     }
 
+    if (images.size() > 1) {
+      Set<OverviewImage> usedImages = new HashSet<>();
+      Set<OverviewImage> childImages = new HashSet<>();
+      for (OverviewImage image : images) {
+        if (image.getLinks().size() > 0) {
+          usedImages.add(image);
+        }
+        for (OverviewLink link : image.getLinks()) {
+          if (link instanceof OverviewImageLink) {
+            usedImages.add(((OverviewImageLink) link).getLinkedOverviewImage());
+            childImages.add(((OverviewImageLink) link).getLinkedOverviewImage());
+          }
+        }
+      }
+      for (OverviewImage image : images) {
+        if (!usedImages.contains(image)) {
+          logger.warn("Image is not referenced in coordinates file: " + image.getFilename());
+        }
+      }
+      if (images.size() > childImages.size() + 1) {
+        logger.warn("In coords.txt, more than one image can be the starting one for 'Show overview',"
+            + " picking at random.");
+      }
+    }
   }
 
   /**