Auto merge of #2797 - staktrace:hittest, r=nical
Auto merge of #2797 - staktrace:hittest, r=nical

Rebuild the hit-tester as part of the scene build.

In order for Gecko/APZ to get useful results from hit-testing, the
hit-test tree needs to stay in sync with the scene and clip-scroll tree.
This patch accomplishes this by rebuilding the hit-tester at the
same time we rebuild or swap in these other structures, rather than at
render time.

r? @mrobinson

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