fix: render thumbnails on uosc side

[po5]

I've isolated what causes the occasional thumbnail/thumbnail border desync.
It's not a performance problem of either thumbfast or uosc.
It's simply because the two don't update at the same time.

I had anticipated this and provided the thumbfast-render script message for those who wanted to implement it.
Turns out it was unusable because mpv wouldn't let you pass the arguments it expects, and the clear command would remove the overlay straight away (undesirable in script render mode).

With these issues fixed, I've implemented it (in a hacky way) in uosc.
What this does is move all thumbnail and border rendering to render().
That way, the overlay is always updated at the same time (or as close to it as mpv will allow) as its border.
This seems to fix any lags between the two elements on my end, be it when moving really fast or when clearing the thumbnail.

Make sure you are on the latest thumbfast commit.

Is this something you would consider doing, or are you fine with the current slightly inaccurate thumbnails with a simpler implementation?