When speeding up or slowing down clips, gcut currently only ever uses the
nearest full frame and doesn't do any blending between frames. I haven't tested
this well, but suspect that with 60fps source footage, one can do nice half
speed slow downs as well as speed ups of the footage without trouble if the
target is 30fps.

Adding the capability of doing higher quality inter frame motion interpolation
based would be possible in the future, but would high quality data the current
might be sufficient.

This is also the method used by gcut when mixing videos of different fps, not
ideal but it should do the job of having video.