[osx] - setvideoresolution needs to be called from mainthread else the o...

...pengl context in windowing can't be accessed - fixes #15115.

Fixes switching videoscreen.screen or videoscreen.resolution from json-rpc which is done from non-mainthread.

@topfs this is for helix - done it as a pr as i don't know if there are any constraints on the TMSG_ defines ...
great catch thx