demux_lavf: improve image detection

This moves the image check to where the number of frames is available of
comparison, which allows not detecting jpg and png videos as images, and
detecting 1-frame gifs as images. This works with the mjpeg and png
videos in the FATE suite, though unfortunately the bmp video is still
detected as an image since it has nb_frames = 0.

aliaspix streams are also now considered images.

Attached pictures are now treated like standalone images, so audio with
attached pictures now has mf-fps as container-fps instead of
unavailable, which makes it consistent with external cover art, which
was already being assigned mf-fps.

Unfortunately images in a codec commonly used for videos are never
detected, and detection was inaccurate even using the now private
codec_info_nb_frames field in AVStream, and mediainfo gets them wrong
too, so I guess it's just a lost cause.

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