At the time screenshot support was added, images weren't refcounted yet,
so screenshots required specialized implementations in the VOs. But now
we can handle these things much simpler. Also see commit 5bb24980.
If there are VOs in the future which can't do this (e.g. they need to
write to the image passed to vo_driver->draw_image), this still could be
disabled on a per-VO basis etc., so we lose no potential performance
advantages.
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