Cache the font information

[fredj]

This is a pull request to improve to label performances.

Instead of parsing the font for each label, a cache is used to store the information.
Adding an element to the dom and calling getComputedStyle forces a reflow and is bad for the performance.

The cache is just and object and is never cleared because I've assumed the it won't get very big.

Thoughts?

[cesium-concierge]

Thanks for the pull request @fredj!

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[hpinkos]

@lilleyse or @IanLilleyT can you review this?

[lilleyse]

@fredj I think I'm good with this change even if the cache doesn't get cleared. There was a similar discussion here about clearing the cache, are any of those ideas actionable?

[fredj]

@lilleyse What about 8060c3a? only the first 256 fonts are saved into the cache

[lilleyse]

Thanks @fredj!

[lilleyse]

I updated CHANGES.md in 5cc6fa6