Clear depth after rendering CB surface.
Clear depth after rendering CB surface.

This avoids depth fighting artifacts between the surface and other
primitives.  This won't cut it for terrain - a more complicated solution
is necessary there - but it makes this branch work like master.

ggetz/cesium,progsung/cesium,esraerik/cesium,ggetz/cesium,aelatgt/cesium,soceur/cesium,esraerik/cesium,kaktus40/cesium,NaderCHASER/cesium,wallw-bits/cesium,omh1280/cesium,hodbauer/cesium,ggetz/cesium,jason-crow/cesium,denverpierce/cesium,YonatanKra/cesium,jasonbeverage/cesium,likangning93/cesium,AnimatedRNG/cesium,kiselev-dv/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,kiselev-dv/cesium,oterral/cesium,progsung/cesium,omh1280/cesium,wallw-bits/cesium,omh1280/cesium,hodbauer/cesium,jason-crow/cesium,CesiumGS/cesium,likangning93/cesium,kiselev-dv/cesium,soceur/cesium,wallw-bits/cesium,YonatanKra/cesium,NaderCHASER/cesium,ggetz/cesium,likangning93/cesium,kaktus40/cesium,emackey/cesium,oterral/cesium,geoscan/cesium,esraerik/cesium,aelatgt/cesium,aelatgt/cesium,hodbauer/cesium,NaderCHASER/cesium,likangning93/cesium,emackey/cesium,YonatanKra/cesium,jason-crow/cesium,denverpierce/cesium,kiselev-dv/cesium,AnimatedRNG/cesium,denverpierce/cesium,jason-crow/cesium,josh-bernstein/cesium,YonatanKra/cesium,wallw-bits/cesium,AnimatedRNG/cesium,emackey/cesium,omh1280/cesium,soceur/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,emackey/cesium,aelatgt/cesium,CesiumGS/cesium,esraerik/cesium,oterral/cesium,geoscan/cesium,jasonbeverage/cesium,denverpierce/cesium,AnimatedRNG/cesium,josh-bernstein/cesium,CesiumGS/cesium,likangning93/cesium