Gave numpy_image a (rows,cols) constructor.

dlib/python/numpy_image.h

@@ -146,6 +146,14 @@ namespace dlib
             assert_is_image<pixel_type>(img);
         }
 
+        numpy_image (
+            long rows,
+            long cols
+        )
+        {
+            set_size(rows,cols);
+        }
+
         numpy_image (
             const py::object& img
         ) : numpy_image(img.cast<py::array>()) {}