Auto merge of #16818 - zombiefungus:fix15638, r=Manishearth

Add default value to layout.threads in prefs.rs (fix #15638)

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Add default value to layout.threads in prefs.rs

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15638  (github issue number if applicable).

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