Fix two build problems with CSS
Fix two build problems with CSS

* `release` didn't minify CSS.
* `minifyCSS` didn't always incorporate the latest CSS source, since it operates in-place.

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