When -pix_fmt designates a BE/LE pixel format, it gets translated into
the native one by av_get_pix_fmt(). This may not always be the best
choice, as the encoder might only support one endianness. In such a
case, explicitly choose the endianness supported by the encoder.
While this is currently redundant with choose_pixel_fmt() in
ffmpeg_filter.c, the latter code will be deprecated in following commits.
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