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/include/opencv2/imgcodecs.hpp

@@ -131,6 +131,14 @@ returns an empty matrix ( Mat::data==NULL ). Currently, the following file forma
  */
 CV_EXPORTS_W Mat imread( const String& filename, int flags = IMREAD_COLOR );
 
+/** @brief Loads and resizes down an image from a file.
+@anchor imread_reduced
+@param filename Name of file to be loaded.
+@param flags Flag that can take values of @ref cv::ImreadModes
+@param scale_denom
+ */
+CV_EXPORTS_W Mat imread_reduced( const String& filename, int flags = IMREAD_COLOR, int scale_denom=1 );
+
 /** @brief Loads a multi-page image from a file. (see imread for details.)
 
 @param filename Name of file to be loaded.