screenshots: select best image format the encoder supports

This matters for png screenshots. We used to hardcode rgb24, but
libavformat's png encoder can do much more. Use the image format list
provided by the encoder, and select the best format from it (according
to the source format).

As a consequence, rgb48 (i.e. 16 bit per component) will be selected if
the source format is e.g. 10 bit yuv. This happens in accordance to
FFmpeg's avcodec_find_best_pix_fmt_of_list() function, which assumes
that 16 bit rgb should be preferred for 10 bit yuv.

This also causes it to print this message in this case:

[ffmpeg] swscaler: full chroma interpolation for destination format 'rgb48be' not yet implemented

I'm not 100% sure whether this is a problem.

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