Reset inView and entry when observed component is unmounted

[chapa]

Fix #303

I'm not 100% sure of the need to use React.useLayoutEffect instead of React.useEffect, but it may be safer since we nee to execute the callback after the node has been removed from the ref by React (maybe after DOM mutations ?).

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[coveralls]

Pull Request Test Coverage Report for Build 1724

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 98.974%

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Totals
Change from base Build 1719:	0.04%
Covered Lines:	107
Relevant Lines:	107

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💛 - Coveralls

[thebuilder]

Hey @chapa, thanks for making a PR for this. I have also been working on the fix myself, but not committed yet.

I would simply clear the state when the ref callback is called with a new (null) node. No need for an extra effect.

[chapa]

Of course !
You solution is much simpler, I didn't think the callback will be called on unmount 😄