libavfilter/vf_fps: Rewrite using activate callback

The old version of the filter had a problem where it would queue up
all of the duplicate frames required to fill a timestamp gap in a
single call to filter_frame. In problematic files - I've hit this in
webcam streams with large gaps due to network issues - this will queue
up a potentially huge number of frames. (I've seen it trigger the Linux
OOM-killer on particularly large pts gaps.)

This revised version of the filter using the activate callback will
generate at most 1 frame each time it is called.