Outgoing edges of nodes should be sorted based on when a call `.stream()` created them

Created by: depau

i = ffmpeg.input("f1.mp4")
ref = ffmpeg.input("f2.mp4")
# BTW `filter_multi_output` is not in __all__, needs #65
s2ref = ffmpeg.filter_([i, ref], "scale2ref").node
scaled = s2ref[0]
ref2 = s2ref[1]

https://ffmpeg.org/ffmpeg-all.html#scale2ref

You would expect the output of scale2ref labeled 0 (scaled) to be the first output of scale2ref (because .stream() was called first), and 1 (ref2) to be the second. However, this doesn't always happen: it's totally up to topo_sort().

I'll try to fix it and send a PR.