Enum for loss returned by `find_best_pix_fmt_of_list`?

[oakaigh]

Currently loss is returned by find_best_pix_fmt_of_list as an int; A PixFmtLoss enum.IntFlag should be easier to work with.

Expose the flags defined here:
https://github.com/FFmpeg/FFmpeg/blob/07ae44a607d86dabb38bb591f28ddf4e49386479/libavutil/pixdesc.h#L397-L404