Frustum class/box intersection testing.
Filter terrain, points, spanners by frustum intersection.
Merge branch 'master' of https://github.com/highfidelity/hifi into metavoxels
Added simple "erase heightfield" tool.
Merge branch 'master' of https://github.com/highfidelity/hifi into metavoxels
Merge pull request #3249 from ey6es/metavoxels

Cull heightfield/points/spanners against frustum, added simple "erase heightfield" tool.