Added image_traits specializations for const types because it makes

certain compiler errors you get when accidentally trying to mutate
a const image a little easier to understand.

dlib/array2d/array2d_generic_image.h

@@ -13,6 +13,11 @@ namespace dlib
     {
         typedef T pixel_type;
     };
+    template <typename T, typename mm>
+    struct image_traits<const array2d<T,mm> >
+    {
+        typedef T pixel_type;
+    };
 
     template <typename T, typename mm>
     inline long num_rows( const array2d<T,mm>& img) { return img.nr(); }

dlib/matrix/matrix_generic_image.h

@@ -19,6 +19,17 @@ namespace dlib
         typedef T pixel_type;
     };
 
+    template <
+        typename T,
+        long NR,
+        long NC,
+        typename MM
+        >
+    struct image_traits<const matrix<T,NR,NC,MM> >
+    {
+        typedef T pixel_type;
+    };
+
     template <
         typename T,
         long NR,