Fix performance bug.
Fix performance bug.

Tile vertex arrays were being recreated every update because the
tile._mode property was not set.

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