DisplayF2Offset change in 2023 version of SMPTE ST 2067-21

[thomasheritage]

https://github.com/bbc/bmx/blob/main/docs/imf_track_files.md says that in the IMF flavour:

add the Display F2 Offset and sets it to 0 for progressive JPEG 2000 and RDD 36 video (--display-f2-offset 0)

This was appropriate for the 2020 version of SMPTE ST 2067-21. However, the 2023 version says for DisplayF2Offset:

Shall be present if interlaced structure is used and should not be present if progressive structure is used.

For now it's probably best to take no action (to avoid backwards compatibility issues, and because this is not significant), but perhaps leave this issue open as a reminder.