adaptivedemux2: Fix download helper with libsoup 3.0.x

libsoup 3.0.x dispatches using a single source attached when the session is created, so we need to create the session with the same context that our download thread is later using.

2.74 or 3.1 will dispatch a response using the context which sent the request. However, for any context other than the one that created the session, this will also create and destroy sources, so there's still some slight performance benefit.

Fixes: #1384 (closed)