Add some comments on how the code should work. Basic idea is first
check input lipsync against audio track. Then check if we can drop the
picture based on output pts and then do same checked in OutputFrame as
some filters can output multiple pictures (yadif2x etc).

simple framerate conversion from 25 worked to 4,5,10,20,23,29.97,50.60.
Not sure if yadif2x and all work fully yet.