It makes sense to treat the presence of a frame duration and the presence
of frame rate metadata identically - because both convey effectively the same
amount of information.
In f121d95 and fa110c3 respectively, this information was stripped by default,
originally to work-around bugs when changing the PTS information of a stream
being fed to some encoders. (See https://trac.ffmpeg.org/ticket/10886)
Later, commit 959b799c restored the ability to preserve the frame rate
medatata via the `strip_fps` option, but this option did not extend to also
include the frame duration.
This commit resolves the scenario by making `frame_rate` and `duration`
handled in a consistent manner, so that the frame rate information is
generally preserved unless explicitly stripped by the user.
While it does regress the exact invocation presented in the trac ticket unless
using `strip_fps=yes`, I consider this an acceptable trade-off, especially in
light of the fact that the `fps` filter also exists and is arguably the better
tool for the task at hand.
8e0de5d811