Terrain snow shading

@VElysianP @akhabbaz @HuYuxin great job with your snow rendering project in CIS 565!

Just like the BabylonJS teams, would you like to contribute it back to Cesium? It could just be the snow applied to the terrain, not the snow fall since I do not know if the snow fall will be fast enough for most Cesium users.

Please open a pull request and someone on the team can walk you through the process and even do some of the work to get it into production shape. For more on contributing to Cesium, see [Opening a Pull Request](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/CONTRIBUTING.md#opening-a-pull-request). Thanks!

https://github.com/HuYuxin/CIS565FinalProjectCesiumSnow/blob/master/CIS565FinalProjectREADME.md

Start by reading the linked CIS565FinalProjectREADME.md and Cesium's CONTRIBUTING.md section on opening a pull request. Determine how the project's terrain snow rendering could be contributed separately from snowfall; done would be a production-ready pull request for terrain snow shading.

Written by the indexing model from the issue text.