Add memory usage based heurisitic for unloading tiles
Add memory usage based heurisitic for unloading tiles

Adding a maximumMemoryUsage limiter which behaves similar to the
maximumNumberOfLoadedTiles property. When the cache exceeds usage, all
unneeded tiles are unloaded.

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