Fall back to delta time when physics can't keep up
Right now, OpenMW slows the simulation down if physics can't keep up with the rest of it, and it does it in a dangerous way that causes a problem that we call the physics death spiral. We should fall back to delta time when physics can't keep up instead. (Slowing down more gracefully would be fine too, but keeping mechanics in sync with physics might be difficult/invasive, so delta time is probably easier.)
Related to #5431 (closed).