Demo recording and playback

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Supposedly a feature of the original DOS versions (there are references to "history buffer", though I've never actually made it work). Besides being a nice feature, being able to record and play back demos could also useful for regression testing.

A nice "stretch goal" would be a mission recorder system with the ability to construct little gameplay videos (see Red Baron's mission recorder feature to understand what I mean)

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Could also be useful for #10, in that demo recordings of high scoring games could be uploaded as proof