Video encoding: forward command-line options to ffmpeg

[aumouvantsillage]

Sozi-export has very limited support for customizing the generated videos:

• It calls ffmpeg with a fixed frame rate.
• The video codec is chosen automatically by ffmpeg from the extension of the output file.

Instead of adding more command-line options to Sozi-export, we could provide a syntax for setting any ffmpeg option.
For instance -F,-r 30 would set the frame rate option (-r) to 30.

Option -b already allows to set the bitrate, but it would be equivalent to -F,-b:v