Fix polytomy bounding box calculation.

[wolfmanstout]

Caused numerical instability when zooming deep (e.g. Citrus reticulata) due to bad choice of graphref.

[wolfmanstout]

Using onezoom.config.debug_bounding_box = 0x01 ...

Before:
After:

[lentinj]

This is approximately undoing 3bc42f5 - did you pull the thread to see we can rely on node.upnode.arcr now?

This might have been solved in the interim, I can't remember off the top of my head so would have to have a rummage.

[wolfmanstout]

Good question, I can check later.

[wolfmanstout]

Reverting 3bc42f5 causes the same problems as initially noted in that commit. At first I thought it was working well, but if I reloaded the page after going to sea cucumbers and then zoomed out with bounding boxes enabled, I could see large bounding boxes when I zoomed out that would eventually disappear. I do not see this issue with this PR.