Grabcut with frozen models (#11339)

* model is not learned when grabcut is called with GC_EVAL

* fixed test, was writing to wrong file.

* modified patch by Iwan Paolucci; added GC_EVAL_FREEZE_MODEL in addition to GC_EVAL (which semantics is retained)

modules/imgproc/include/opencv2/imgproc.hpp

@@ -386,7 +386,9 @@ enum GrabCutModes {
     automatically initialized with GC_BGD .*/
     GC_INIT_WITH_MASK  = 1,
     /** The value means that the algorithm should just resume. */
-    GC_EVAL            = 2
+    GC_EVAL            = 2,
+    /** The value means that the algorithm should just run the grabCut algorithm (a single iteration) with the fixed model */
+    GC_EVAL_FREEZE_MODEL = 3
 };
 
 //! distanceTransform algorithm flags