Add Support for Swithching FPS Between 30 and 60 on Run Time

It would be good to support switching the rendering into 30FPS rendering (as opposed to default 60FPS). It can be achieved with Qt by using QSurfaceFormat::setSwapInterval API. But looks like this API call only works before the window and OpenGL context are created. After that, it has no effect, which doesn’t satisfy the need of evaluating FPS changing from 60 to 30.