Use efficient way in `frame_iterator`

[coord-e]

frame_iterator is not a random access iterator, so it can iterate without calling Motion::frame_at in each forwards.

[coord-e]

Proposed APIs:

• Motion Motion::resampled(double fps)

and Motion::frames would be:

ConstKeyframeRange Motion::frames(double fps) const {
   return this->resampled(fps).const_keyframes();
}

[coord-e]

insufficient but not required. not in next version

[coord-e]

also resampled would be very ineffective