vo_gpu_next: implement VOCTRL_SCREENSHOT

Somewhat annoying but still relatively straightforward. There are
several ways to approach this, but I settled on reusing the pl_queue as
a cheap way to get access to the currently mapped frame. This saves us
from having to process `vo_frame` at all, and also avoids any overhead
from re-uploading the same frame twice.

(However maybe there's some circumstance in which `vo_frame` needs to be
queried/updated first, to get a screenshot of the correct frame? I'm not
sure.)

I also had the option of going with either pl_render_image() on the
extract pl_frame, or just calling pl_render_image_mix directly on it. I
went for the latter, because in the optimal case, this allows the
rendered frame to be directly retrieved from the cache, actually
entirely avoiding any sort of recompute overhead. This makes e.g. ctrl+s
during playback essentially free. (Except for the download cost,
obviously)

It would be even neater if we could make this VOCTRL asynchronous and
thereby leverage libplacebo's asynchronous download capabilities. But oh
well. That will have to wait for a sufficiently rainy day.

Closes #9388

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