Point, billboard, and labels roadmap

- [ ] Optimize ray/terrain intersection when getting height. At least add a 2x2 or 4x4 grid, if not an octree/quadtree that is a few levels deep. That will probably take a day and produce a 10x gain. Also consider temporal coherence, adjacency info, and simplifying the math for the case when the ray is along (well opposite) the ellipsoid normal.
- [ ] Clamp points to terrain.
- [ ] Billboard ground shadows to give a better sense of placement. These could be faked like the Milktruck shadows. Perhaps have a sparse texture with the longitude/latitudes and then do a simple blurred black circle at each point in the terrain pass.
- [ ] Multipass with transform-feedback or occlusion queries (CC #797), e.g., we could determine if a billboard is `x%` visible in one pass then cull it in the next pass to be more precise than VTP.

No files or tests are named. Start by reading the terrain pass and the related multipass/transform-feedback or occlusion-query discussion in CC #797, then split the four roadmap items into separately scoped issues; each is done only when its performance, terrain-clamping, shadow, or visibility result has a defined validation.

Written by the indexing model from the issue text.