spheres: move for remaining time after collision
spheres: move for remaining time after collision

When a collision occurs during the time frame dt, colliding objects
are moved to the point of collision at some time t < dt, and assigned
a new velocity. When subsequently moving these objects during this
frame, apply that velocity only for the remaining time (dt - t).

kfish/dreambuggy,kfish/dreambuggy,here4/here4,kfish/dreambuggy,kfish/dreambuggy