Rotation should still work, if mouse is not hovering over model

[hannobraun]

The model is always rotated around the focus point, the point on the model that the mouse points at. This is useful when you're zoomed in closely, inspecting a specific detail.

Currently, if there is no focus point (because the mouse is not point on the model), then rotating doesn't work. This can be confusing. Rotating without a focus point should still work, by using either the coordinate system's origin or the center of the model's bounding volume as a fallback.