Option to clamp point cloud points to terrain

Requested on the forum: https://groups.google.com/forum/#!topic/cesium-dev/Wb4d5ud7nFw

> I have a pointcloud that spans the globe but some of the points clip below the surface. I know this would be an edge-case for point cloud support but I would love the ability to have points conditionally work like GroundPrimitive and move to the top of the surface when it is clipped.
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> Currently my code works as follow: https://pastebin.com/F67gwjc0 By sampling the terrain and then moving the point before load if it is below the surface. The problem with this solution is loading becomes very slow and is not efficient in adjusting points.
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> Overall I would like the ability to handle points living below the surface. This could be done by adjusting the terrain or the points as long as the hidden points are visible. My other idea would be to test the point against cesiums terrain before generating the tile and adding a meta property that will be added on load.

Start by reviewing the linked forum request and the Pastebin example to understand the current terrain-sampling workaround and the requested point-cloud behavior. The issue does not name source files or tests; done would require an agreed implementation that keeps points below terrain visible without the reported slow loading and inefficient adjustment.

Written by the indexing model from the issue text.