Added image_dataset_file::shrink_big_images(). So now load_image_dataset() can

load a dataset of high resolution files into a user requested lower resolution.

dlib/data_io/load_image_dataset_abstract.h

@@ -36,6 +36,7 @@ namespace dlib
                   This means that, initially, all boxes will be loaded.  Therefore, for all
                   possible boxes B we have:
                     - #should_load_box(B) == true
+                - #box_area_thresh() == infinity
         !*/
 
         const std::string& get_filename(
@@ -115,6 +116,26 @@ namespace dlib
                         - returns false
         !*/
 
+        image_dataset_file shrink_big_images(
+            double new_box_area_thresh = 150*150
+        ) const;
+        /*!
+            ensures
+                - returns a copy of *this that is identical in all respects to *this except
+                  that #box_area_thresh() == new_box_area_thresh
+        !*/
+
+        double box_area_thresh(
+        ) const;
+        /*!
+            ensures
+                - If the smallest non-ignored rectangle in an image has an area greater
+                  than box_area_thresh() then we will shrink the image until the area of
+                  the box is about equal to box_area_thresh().  This is useful if you have
+                  a dataset containing very high resolution images and you don't want to
+                  load it in its native high resolution.  Setting the box_area_thresh()
+                  allows you to control the resolution of the loaded images.
+        !*/
     };
 
 // ----------------------------------------------------------------------------------------