Fixed resize_image() not using the optimized code path in some cases. In

particular, it wasn't being used in some parts of the Python API.  So this
change makes the HOG detector run like 10% faster for instance.

dlib/image_transforms/interpolation.h

@@ -228,12 +228,6 @@ namespace dlib
 
     class interpolate_bilinear
     {
-        template <typename T>
-        struct is_rgb_image 
-        {
-            const static bool value = pixel_traits<typename T::pixel_type>::rgb;
-        };
-
     public:
 
         template <typename T, typename image_view_type, typename pixel_type>
@@ -330,11 +324,6 @@ namespace dlib
 
     class interpolate_quadratic
     {
-        template <typename T>
-        struct is_rgb_image 
-        {
-            const static bool value = pixel_traits<typename T::pixel_type>::rgb;
-        };
 
     public:
 
@@ -907,11 +896,12 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename image_type
+        typename image_type1,
+        typename image_type2
         >
-    typename enable_if<is_rgb_image<image_type> >::type resize_image (
-        const image_type& in_img_,
-        image_type& out_img_,
+    typename enable_if_c<is_rgb_image<image_type1>::value && is_rgb_image<image_type2>::value >::type resize_image (
+        const image_type1& in_img_,
+        image_type2& out_img_,
         interpolate_bilinear
     )
     {
@@ -922,14 +912,14 @@ namespace dlib
             << "\n\t is_same_object(in_img_, out_img_):  " << is_same_object(in_img_, out_img_)
             );
 
-        const_image_view<image_type> in_img(in_img_);
-        image_view<image_type> out_img(out_img_);
+        const_image_view<image_type1> in_img(in_img_);
+        image_view<image_type2> out_img(out_img_);
 
         if (out_img.size() == 0 || in_img.size() == 0)
             return;
 
 
-        typedef typename image_traits<image_type>::pixel_type T;
+        typedef typename image_traits<image_type1>::pixel_type T;
         const double x_scale = (in_img.nc()-1)/(double)std::max<long>((out_img.nc()-1),1);
         const double y_scale = (in_img.nr()-1)/(double)std::max<long>((out_img.nr()-1),1);
         double y = -y_scale;