Minor optimization: add shortcut to in-place image resize if size_scale is 1 (#2076)

dlib/image_transforms/interpolation.h

@@ -1062,7 +1062,7 @@ namespace dlib
             << "\n\t Invalid inputs were given to this function."
             << "\n\t size_scale:  " << size_scale
             );
-
+        if (size_scale == 1.0) return; // no need to do anything at all
         image_type temp;
         set_image_size(temp, std::round(size_scale*num_rows(img)), std::round(size_scale*num_columns(img)));
         resize_image(img, temp);

dlib/image_transforms/interpolation_abstract.h

@@ -435,6 +435,7 @@ namespace dlib
                 - #img.nr() == std::round(size_scale*img.nr())
                 - #img.nc() == std::round(size_scale*img.nc())
                 - #img == a bilinearly interpolated copy of the input image.
+            - Returns immediately, if size_scale == 1.0
     !*/
 
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