add default file to load in case sample was run without command line arguments

samples/gpu/opengl.cpp

@@ -14,6 +14,9 @@ int main()
     #define NOMINMAX 1
     #include <windows.h>
 #endif
+#if defined(_WIN64)
+    #include <windows.h>
+#endif
 
 #if defined(__APPLE__)
     #include <OpenGL/gl.h>
@@ -55,16 +58,19 @@ void draw(void* userdata)
 
 int main(int argc, char* argv[])
 {
+    string filename;
     if (argc < 2)
     {
         cout << "Usage: " << argv[0] << " image" << endl;
-        return -1;
+        filename = "../data/lena.jpg";
     }
+    else
+        filename = argv[1];
 
-    Mat img = imread(argv[1]);
+    Mat img = imread(filename);
     if (img.empty())
     {
-        cerr << "Can't open image " << argv[1] << endl;
+        cerr << "Can't open image " << filename << endl;
         return -1;
     }