This is pretty imperfect, but it's just a demo.
The main purpose is clarifying how and when to get the video size.
In theory, retrieving the properties this way has a race condition:
after reading dwidth, the video could resize again. But the worst that
can happen are mismatching dwidth/dheight values, and the
MPV_EVENT_VIDEO_RECONFIG event would be immediately received again,
which would fix the mismatch. You could read the full video-out-params
property to absolutely avoid it, but it's not worth the trouble.
A bit verbose, but less misleading. In most cases, the API user probably
actually wants mpv_terminate_destroy() instead, so the less-useful
function shouldn't have a simnpler name anyway.
