Fix segfault when accessing FilterContext after Graph object has left scope

[coredumped67]

Hello. I am experiencing a segfault when performing filter operations due to libav accessing an invalid Graph object. See the following example:

def make_filter(stream1: VideoStream, stream2: VideoStream) -> tuple[FilterContext, FilterContext, FilterContext]:
    graph = Graph()
    filter_source1 = graph.add_buffer(stream1)
    filter_source2 = graph.add_buffer(stream2)

    alphamerge = graph.add('alphamerge')
    filter_source1.link_to(alphamerge, 0, 0)
    filter_source2.link_to(alphamerge, 0, 1)

    filter_sink = graph.add('buffersink')
    alphamerge.link_to(filter_sink)

    graph.configure()
    return filter_source1, filter_source2, filter_sink
    # graph loses reference and gets free'd


if __name__ == '__main__':
    stream1, stream2 = get_streams()

    source1, source2, sink = make_filter(stream1, stream2)

    # Sources and sinks contain a weakref to the graph, and the
    # validity of said graph isn't properly checked during operations
    # such as FilterContext.push(...), resulting in a segfault.
    do_filter_stuff(source1, source2, sink)

My solution is to remove the weakref entirely, but I'm not sure how much of an impact this has on memory usage. I'm a little in over my head poking around in FFMPEG source, so I'm open to suggestions if this approach isn't feasible.

[WyattBlue]

This PR might bring regression for a memory leak problem. #1439

[coredumped67]

Hmmm. We could keep the weakref and check it's validity every time we cross the libav boundary. Users could still experience weird lifetime problems, but at least those would result in an exception rather than a segfault. Perhaps a "better" solution would be to perform all filter operations on the graph object itself. Something like this:

def make_filter(stream: VideoStream) -> Graph:
    graph = Graph()
    graph.add_buffer(stream, 'my-source')
    graph.add('crop', 'crop-to-thumbnail', '100:100')
    graph.link_to('my-source', 'crop-to-thumbnail', 0, 0)
    graph.add('buffersink', 'my-sink')
    graph.link_to('crop-to-thumbnail', 'my-sink', 0, 0)
    return graph
    
filter = make_filter(stream)
filter.push('my-source', frame)

I don't think the implementation would have to change too much. FilterContext would no longer be exposed and Graph would maintain a mapping of filter names to objects. But this approach would result in breaking changes :(