added fetch as a counter-part to feed to V4D

modules/v4d/include/opencv2/v4d/v4d.hpp

@@ -193,6 +193,12 @@ public:
      * Called to feed an image directly to the framebuffer
      */
     CV_EXPORTS void feed(cv::InputArray& in);
+    /*!
+     * Fetches a copy of frambuffer
+     * @return a copy of the framebuffer
+     */
+    CV_EXPORTS InputOutputArray fetch();
+
     /*!
      * Called to capture to the framebuffer from a #cv::viz::Source object provided via #V4D::setSource().
      * @return true if successful.

modules/v4d/src/v4d.cpp

@@ -205,6 +205,15 @@ void V4D::feed(cv::InputArray& in) {
         });
 }
 
+InputOutputArray V4D::fetch() {
+        cv::UMat frame;
+
+        fb([frame](cv::UMat& framebuffer){
+            framebuffer.copyTo(frame);
+        });
+        return frame;
+}
+
 bool V4D::capture() {
     return this->capture([&](cv::UMat& videoFrame) {
         if (source_.isReady())