Add support for water masks from quantized-mesh terrain

[j9liu]

Feature

As seen on the forum:

• https://community.cesium.com/t/how-do-i-set-a-water-mask-material/22017
• https://community.cesium.com/t/unity-cesium-water-system/36171/2
• https://community.cesium.com/t/custom-water-cesium/26288/3
• https://community.cesium.com/t/how-do-i-use-the-custom-materials/37462/10

[JanikCodes]

bump