Fix OSX not starting matches following rP21264.
That commit illegaly polled SDL events from a different thread which for an unknown reason did not error on Windows and GNU/linux.

Refs #4822 / D1304, similarly rP19160.

Differential Revision: https://code.wildfiregames.com/D1484
Previous iteration tested on OSX and some comments with regards to letter case by Vladislav (refs rP21817, rP21816).