Terrain 'pokes through' large corridors

Seen on the [CZML corridor sandcastle](http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=CZML%20Corridor.html&label=CZML) with terrain on.

Terrain clips through the red corridor for tall mountains in the Rockies.

Start with the linked CZML Corridor Sandcastle example and enable terrain, then reproduce the clipping on tall Rocky Mountain terrain. Trace the corridor and terrain rendering entry points involved in that example; done means the red corridor no longer has terrain showing through it in the reproduction case.

Written by the indexing model from the issue text.