In order to keep all of the tragic implementation detail for this function in a single class, I originally put the structs and the thread proc inside the function.
The thread proc was implemented as a lambda which implicitly converts to a function pointer.
Unfortunately, it seems MinGW can't implicitly convert this lambda to a function with the stdcall calling convention.
This is why we can't have nice things.
To deal with this, refactor it into a class so that we can define the thread proc as a static function instead of a lambda.
All of the methods are static; the class is just used to contain the implementation details.
Differential Revision: https://phabricator.services.mozilla.com/D212391
5c6a8e23d4