Engine as a video codec (video decoder)

[JPL]

I've never been under the impression that demo playback is long-term stable across versions in GZDoom; the engine is complex enough and evolves constantly enough, there are so many things I can imagine would cause demos recorded a year or more ago to desync.
Traditional input-recording style (LMP) demos are more useful for a Doom port with an essentially frozen feature set and/or rigorous back compat standards, which is why PrBoom+ and Chocolate Doom are the main ports people use for recording and playing back, for example, competitive demos.
It's an interesting idea but this project doesn't seem like the best fit for it.