Point, billboard, and labels roadmap

[pjcozzi]

• 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 WebGL 2 and WebGL Extensions Roadmap #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.

[emackey]

See also some ideas re-thinking the way points/billboards/labels could be separated from the 3D scene, in #2694 (comment)