Tree flickers oddly during animation

[sidneymbell]

Looks like maybe an edge case for small trees, where the default label rendering logic is creating an odd flicker.

[trvrb]

Thanks @sidneymbell! This is indeed quite jarring: https://nextstrain.org/ncov?animate=2019-11-25,2020-02-01,0,0,30000. I think the best way to deal with this will be to hide tip labels when animating. I don't think we did testing previously with small trees + animation.