Mesh compression

Hi all,

We have been working with the Google Draco team to add a mesh and point compression extension to glTF, see the extension, #874, and general info on Draco.

This may have a broad impact on Cesium since glTF is the primary payload in 3D Tiles, so Draco mesh compression may become widely used for everything from 3D models to buildings to point clouds to terrain.

If you are interested in this area, please help review the extension: #874

Thanks,

Patrick