add const keywords to gridwidget constructor

modules/viz/include/opencv2/viz/widgets.hpp

@@ -123,7 +123,7 @@ namespace cv
         class CV_EXPORTS GridWidget : public Widget3D
         {
         public:
-            GridWidget(Vec2i dimensions, Vec2d spacing, const Color &color = Color::white());
+            GridWidget(const Vec2i &dimensions, const Vec2d &spacing, const Color &color = Color::white());
         };
 
         class CV_EXPORTS Text3DWidget : public Widget3D

modules/viz/src/shape_widgets.cpp

@@ -440,7 +440,7 @@ template<> cv::viz::PolyLineWidget cv::viz::Widget::cast<cv::viz::PolyLineWidget
 ///////////////////////////////////////////////////////////////////////////////////////////////
 /// grid widget implementation
 
-cv::viz::GridWidget::GridWidget(Vec2i dimensions, Vec2d spacing, const Color &color)
+cv::viz::GridWidget::GridWidget(const Vec2i &dimensions, const Vec2d &spacing, const Color &color)
 {
     // Create the grid using image data
     vtkSmartPointer<vtkImageData> grid = vtkSmartPointer<vtkImageData>::New();