When added to a graph, filter::Context can't be manipulated if another is added

[VincentJousse]

To reproduce, use the transcode-audio example and :

• Create a variable to store the first filter context.
• Create the second filter context.
• Do anything with the first.

let mut filter_context = filter.add(&filter::find("abuffer").unwrap(), "in", &args)?;
filter.add(&filter::find("abuffersink").unwrap(), "out", "")?;

filter_context.set_channel_layout(decoder.channel_layout());

The compiler complains about the Graph lifetime.