[core] look for loaded parent tiles in cache

[ansis]

fix #4047

port mapbox/mapbox-gl-js#2158

When the required tiles aren't loaded yet it tries to find parent tiles that area loaded that a cover the area. It used to just look for parents in the set of previously rendered tiles. There are many cases where the parents aren't in that set but they are in the cache.

This reduces the amount of the viewport not covered by tiles. It makes tile loading look a lot faster.

It helps a lot when you:

zoom in and the pan around
zoom in quickly and then zoom out more slowly

👀 @jfirebaugh