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