SKY: Possible fix for crash when changing scaler
|Beneath a Steel Sky
ScummVM sometimes crashes if you change scaler during the CD intro. I think that is because update_screen() is being called from two different threads, without any safety net against races. This patch attempts to fix that, but I'm a bit uncertain about exactly which functions need to be guarded so it may be slightly wrong and/or incomplete.
Ticket imported from: #726713. Ticket imported from: patches/304.