Generalize tile selection beyond per-frame view frustums

I'm coming from the direction of a possible implementation of a yet-to-be-finalized feature. Basically, if we support vector features, then it makes most sense to associate the vector features displayed "over" a particular tile with that tile when it is loaded, i.e., before it's handed off to a client's `prepareInLoadThread()` function. In this way a client can add data to its render resources that permit the features to be rendered "over" the other tile geometry.

Vector features are quite likely to be stored in a 3D Tiles tileset, either by way of extensions or in a future 3D Tiles version.

Therefore, it would be very useful to search a "secondary" tileset when a tile is loaded, extract any feature data found at the appropriate LOD, and furthermore initiate loading of tile children if SSE for features isn't met.

I think it would awkward to use the current tileset update machinery to do this.

This is a generalization of selecting and loading raster data for tiles.