Added missing requirement

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403971

dlib/image_transforms/assign_image_abstract.h

@@ -97,6 +97,7 @@ namespace dlib
             - image_type == is an implementation of array2d/array2d_kernel_abstract.h
             - x_border_size >= 0
             - y_border_size >= 0
+            - pixel_traits<typename image_type::type> is defined  
         ensures
             - #img.nc() == img.nc()
             - #img.nr() == img.nr()