Make it possible to export to video via CLI

[dbakker]

Currently the only way to export to video is to use the GUI.

It would be fantastic to also be able to export to mp4 via the CLI, similar to the deprecated sozi-to-video command. However with one big difference: That it is based on the JSON file (and regenerates the HTML) so that scripts can change the JSON and regenerate the video.

Examples of external scripts that could make use of the CLI:

• Change an SVG or linked image, update all videos that use it
• Split a presentation up into multiple videos, one for each frame+transition
• Add narration to a presentation (record the narration separately per slide, update the JSON with the timings, regenerate video)
• Have separate sozis share frames. Make a script that syncs the frames over the sozis + regenerates the videos

Having a CLI interface like this would be perfect:
sozi export-video presentation.sozi.json --output presentation.mp4

For now the exports can be done manually one by one after updating the JSON, but it is time consuming and error prone.