Add const to getViewerPose()

getViewerPose() doesn't modify an object of the class so it can be
made const. It also makes this method consistent with other getters
in the class as they are defined as const.

modules/viz/src/vizimpl.cpp

@@ -509,7 +509,7 @@ void cv::viz::Viz3d::VizImpl::setViewerPose(const Affine3d &pose)
 }
 
 /////////////////////////////////////////////////////////////////////////////////////////////
-cv::Affine3d cv::viz::Viz3d::VizImpl::getViewerPose()
+cv::Affine3d cv::viz::Viz3d::VizImpl::getViewerPose() const
 {
     vtkCamera& camera = *renderer_->GetActiveCamera();
 

modules/viz/src/vizimpl.hpp

@@ -88,7 +88,7 @@ public:
     void resetCamera();
 
     void setViewerPose(const Affine3d &pose);
-    Affine3d getViewerPose();
+    Affine3d getViewerPose() const;
 
     void convertToWindowCoordinates(const Point3d &pt, Point3d &window_coord);
     void converTo3DRay(const Point3d &window_coord, Point3d &origin, Vec3d &direction);