Function animates_at 

fn animates_at(
    dim: &Dim,
    walk: Option<Sequence>,
    content: &[u8],
) -> Result<bool>

Whether this source yields an animated variant at these dimensions.

Only a picture whose header says it holds a frame sequence reaches the encoder, so a still never pays a decode that would yield one frame and be thrown away, and a video is excluded by the same test since its container is none of the three that hold frames. A size the request cannot improve on is passed through whole below rather than re-encoded. The caller takes the walk and reads it again at that passthrough, and hands None where the feature is off.