Fix computed value computation

[npm1]

When determining whether the node can be considered unstable, we need to consider not just the node but also its ancestors, since computed value does not account for inheritance. Fixes #61

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[skobes-chromium]

It looks like visibility is inherited, and can be overridden. Should we do this for opacity only?

https://output.jsbin.com/cevoned/quiet

[npm1]

Ahh interesting, done.

[dholbert]

The new spec text looks good -- but let me correct the motivation/reasoning a bit. The commit message said:

[..] since computed value does not account for inheritance.

In fact, the computed value does account for inheritance; and the scenario that we're addressing here doesn't have to do with opacity inheriting or not-inheriting.

The reason that we have to check ancestors (for opacity) is that opacity simply has its effect by changing the painting of the element's whole subtree (by e.g. creating a GPU layer for the element, and then configuring the opacity of that layer on the GPU). So this is why opacity:0 on any ancestor-box will trivially mean that we can be sure that our own box is guaranteed not to paint, regardless of our own box's opacity value.