Made the OpenCV tools #include the right OpenCV headers so the user doesn't need to.

dlib/opencv/cv_image.h

@@ -3,6 +3,8 @@
 #ifndef DLIB_CvIMAGE_H_
 #define DLIB_CvIMAGE_H_
 
+#include <opencv2/core/core.hpp>
+#include <opencv2/core/types_c.h>
 #include "cv_image_abstract.h"
 #include "../algs.h"
 #include "../pixel.h"

dlib/opencv/cv_image_abstract.h

@@ -3,6 +3,8 @@
 #undef DLIB_OPENCV_IMAGE_AbSTRACT_H_
 #ifdef DLIB_OPENCV_IMAGE_AbSTRACT_H_
 
+#include <opencv2/core/core.hpp>
+#include <opencv2/core/types_c.h>
 #include "../algs.h"
 #include "../pixel.h"
 

dlib/opencv/to_open_cv.h

@@ -3,6 +3,7 @@
 #ifndef DLIB_TO_OPEN_Cv_Hh_
 #define DLIB_TO_OPEN_Cv_Hh_
 
+#include <opencv2/core/core.hpp>
 #include "to_open_cv_abstract.h"
 #include "../pixel.h"
 #include "../matrix/matrix.h"

dlib/opencv/to_open_cv_abstract.h

@@ -3,6 +3,7 @@
 #undef DLIB_TO_OPEN_Cv_ABSTRACTh_
 #ifdef DLIB_TO_OPEN_Cv_ABSTRACTh_
 
+#include <opencv2/core/core.hpp>
 #include "../pixel.h"
 
 namespace dlib