dynamic step size adjustment

[BenKnorlein]

To get the best performance it would be nice to have 2 mechanisms for adjusting the number of the step size to have the best accuracy for the used garphics device.

a) By using a slider in the options
b) An automatic adjustment by checking the current framerate. If the framerate is below it a threshold the step size would increase, if above it would decrease. For VR the targeted framerate should be at least 30fps. However, for desktop display it could be much less.

It would also be nice to display the fps somewhere. At least for debugging