Document known bugs with the Multi-Threaded thread model project setting

[Calinou]

• Add a warning on project startup when using the Multi-Threaded option.
• Improve command line validation for the --render-thread CLI argument.

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• See Crash when Thread Model is set to Multi-Threaded  #61650.

[YuriSizov]

I would like someone from @godotengine/core to sign off on the main.cpp changes.

[RandomShaper]

I'm wondering if it wouldn't be better that, in case separate thread is requested, the engine forces single-safe and prints a warning telling about the "downgrade."

[Calinou]

I'm wondering if it wouldn't be better that, in case separate thread is requested, the engine forces single-safe and prints a warning telling about the "downgrade."

On the other hand, we may want to fix this option to be eventually usable in the long term 🙂

So that's why I wouldn't prevent using it entirely.

[RandomShaper]

What I mean is, multi-threaded is kept as a pickable option, but until it's fixed, the engine assumes the closest safe option and informs the user about the matter.

[Riteo]

@RandomShaper tbf I'm pretty divided on this one: on one hand it would be the most consistent and useful choice but I'm seriously worried that once the thing actually becomes useful that people with it on (but what previously was due to this actually single thread mode) would eventually get hard-to-debug issues with it.

[akien-mga]

Thanks!