Reduce label jumping during variable label placement

[tristen]

I've been using the new text-variable-anchor and text-radial-offset properties and I've noticed label re-anchoring has a less desirable visual effect from default label collision.

On first render and subsequent zooms, there's a lot of default label placement then a flicker of recalculations as labels detect their proximity against one another and anchor to different positions.

I'd expect detection to happen behind the scenes. Labels would not be rendered until they know the valid anchor point.

Here's a jsFiddle that demonstrates the behaviour: https://jsfiddle.net/9ub4hdry/7/embedded/result/?username=tristen