dshow: Add ability to save direct show graph

modules/videoio/src/cap_dshow.cpp

@@ -2936,6 +2936,22 @@ int videoInput::start(int deviceID, videoDevice *VD){
     DebugPrintOut("SETUP: Device is setup and ready to capture.\n\n");
     VD->readyToCapture = true;
 
+    // check for optional saving the direct show graph to a file
+    const char* graph_filename = getenv("OPENCV_DSHOW_SAVEGRAPH_FILENAME");
+    if (graph_filename) {
+        size_t filename_len = strlen(graph_filename);
+        std::vector<WCHAR> wfilename(filename_len + 1);
+        size_t len = mbstowcs(&wfilename[0], graph_filename, filename_len  + 1);
+        CV_Assert(len == filename_len);
+
+        HRESULT res = SaveGraphFile(VD->pGraph, &wfilename[0]);
+        if (SUCCEEDED(res)) {
+            DebugPrintOut("Saved DSHOW graph to %s\n", graph_filename);
+        } else {
+            DebugPrintOut("Failed to save DSHOW graph to %s\n", graph_filename);
+        }
+    }
+
     //Release filters - seen someone else do this
     //looks like it solved the freezes