Flickering symbol layers when tiles are refreshed

[jfirebaugh]

Tile layout is a two step process: first layout is performed for all non-symbol layers, then symbol layers are placed. Upon completion of the first step, GeometryTile sets its buckets to the result of that step, which will not contain any symbol buckets. Upon completion of the second step, it merges in symbol buckets.

When cached tiles are revalidated and new data is available, or when the tile layout needs to be refreshed due to runtime styling changes, the result of tile layout is intended to seamlessly replace the previously existing layout. However, due to the way that GeometryTile is currently implemented, any delay between the two steps can lead to flickering symbol layers, because previously existing symbol buckets are effectively discarded upon completion of the first step, and not replaced until the second step completes.

GeometryTile could avoid this effect if it kept symbol and non-symbol buckets segregated, discarding prior symbol buckets only when new symbol buckets became available.

[jfirebaugh]

This is the first bullet point in #7026 (comment).