Fix window resizing not always working

[ts-korhonen]

Summary

Window resizing doesn't always register. It's most noticeable on changing scaling factor. This is because int doresize is used from multiple threads to communicate the need to resize the window. This sometimes leads to lost changes of it's value when it happens concurrently.

The fix here is to replace it with atomic_flag which is performant way to deal with this kind of concurrency problem without introducing locks or mutexes.

Checklist

• Closes #xxx
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set
  • I have opened a roms pull request - https://github.com/86Box/roms/pull/changeme/

References

Provide links to datasheets or other documentation that helped you implement this pull request.