media_player: add an API entry to enable/display the OSD rendering

By default the VLC OSD (play/pause/mute logos, volume slider, seek slider) is enabled. This may not be welcome in libvlc apps that just want the video to be rendered and/or show these things their own way.

We need a way to enable/disable these OSD features, either individually or as a whole.

We may also change the default value as most of the time host apps don't want this. This is mostly useful in the context of the VideoLAN player(s).