new function imread_reduced()

by this new function we can set libjpeg "scale_denom" parameter and load jpeg images scaled 1/2 - 1/4 - 1/8
other image formats resized after loading

modules/imgcodecs/src/precomp.hpp

@@ -47,6 +47,7 @@
 #include "opencv2/core/utility.hpp"
 #include "opencv2/core/private.hpp"
 
+#include "opencv2/imgproc.hpp"
 #include "opencv2/imgproc/imgproc_c.h"
 #include "opencv2/imgcodecs/imgcodecs_c.h"