Fixed load_image_dataset()'s skip_empty_images() option. It wasn't skipping

images that only have ignore boxes when you load into mmod_rects like it should
have been.

dlib/data_io/load_image_dataset.h

@@ -188,6 +188,20 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
+    namespace impl
+    {
+        size_t num_non_ignored_boxes (const std::vector<mmod_rect>& rects)
+        {
+            size_t cnt = 0;
+            for (auto& b : rects)
+            {
+                if (!b.ignore)
+                    cnt++;
+            }
+            return cnt;
+        }
+    }
+
     template <
         typename array_type
         >
@@ -235,7 +249,7 @@ namespace dlib
                 }
             }
 
-            if (!source.should_skip_empty_images() || rects.size() != 0)
+            if (!source.should_skip_empty_images() || impl::num_non_ignored_boxes(rects) != 0)
             {
                 load_image(img, data.images[i].filename);
                 if (rects.size() != 0)  

dlib/data_io/load_image_dataset_abstract.h

@@ -51,8 +51,9 @@ namespace dlib
         ) const;
         /*!
             ensures
-                - returns true if we are supposed to skip images that don't have any boxes
-                  to load when loading an image dataset using load_image_dataset().
+                - returns true if we are supposed to skip images that don't have any
+                  non-ignored boxes to load when loading an image dataset using
+                  load_image_dataset().
         !*/
 
         image_dataset_file boxes_match_label(