MTA was checked in the Open function. As this function is called from libvlc thread, this implies that a user of libvlc should have its own thread in MTA which may not be possible for various reasons.
- MuteSet, VolumeSet, Play, DeviceSelect, Close callback don't require MTA
- Flush, Pause and Start callbacks still require MTA, but as these functions are called from VLC internal threads this is a lesser issue
(cherry-picked from 2fa81b6f)