This was much easier than I thought :P.
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This is a fix for a crash on Mac OS Catalina. D2019 reported issues with video setup in a separate thread, but it turns out it actually crashes, and D2019 is not enough to fix it (in fact I think it's partly the issue, by virtue that further graphics call must be on the initialiser thread).
The simplest solution I find to fixing these issues, which actually date back to #500, is to not put Atlas and the game in separate threads.
The reason for this architecture seems to be found [[ https://wildfiregames.com/forum/index.php?/topic/7444-u-turn-wxwidgets-scenario-editor/ | here ]] (staff forum thread). Basically, it was reasoned that it would help the editor not lag even if the game itself lagged. While true, I find this argument not extremely compelling given the fact that one of our supported OS crashes.
The implementation of messages and such in Atlas makes it rather easy to not actually do threading. With the need for threading removed, a (much?) simpler implementation is likely possible, but this is a working first step (cleanup pending).
In my quick tests, I haven't noticed anything broken or obviously not working. I haven't yet bothered to reimplement "renderer incremental loop".
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Makes D2019 and D1922 irrelevant (and ergo fixes #5470)