demux_lavf: route nested HTTP opens through the curl backend

Hook AVFormatContext.io_open so HLS/DASH segment fetches and other
nested FFmpeg I/O go through stream_http when libcurl is available.
This lets segments inherit the connection reuse, HTTP/2 multiplexing,
mid-stream resume and conditional compression of the top-level stream.

Falls back to FFmpeg's default I/O for non-HTTP URLs and when the curl
backend is not built in.