Fixing parameters reloaded when testing camera. Also fixed app freezing when closing Preferences dialog after using test camera dialog

tools/DatabaseViewer/main.cpp

@@ -53,6 +53,12 @@ int main(int argc, char * argv[])
     QSurfaceFormat::setDefaultFormat(QVTKRenderWidget::defaultFormat());
 #endif
 
+	// Recommended by VTK when using QVTKOpenGLNativeWidget (a QOpenGLWidget): let all VTK
+	// render widgets share a single OpenGL context, which is needed for correct rendering
+	// when render widgets live in / move across multiple top-level windows. Must be set
+	// before QApplication is constructed.
+	QApplication::setAttribute(Qt::AA_ShareOpenGLContexts);
+
 #ifdef RTABMAP_PYTHON
 	rtabmap::PythonInterface pythonInterface;
 #endif