Added support for loading a watermask for Quantized-Mesh terrain tiles.

These changes add support for the new quantized-mesh extension, watermask.  This extension is meant to be a drop-in replacement for the heightmap-1.0 terrain watermask, and shares the same format; 1 byte if all land/water, 64k mask for tiles with a mix of land and water.
Can we review and [publish](https://cesiumjs.org/data-and-assets/terrain/formats/quantized-mesh-1.0.html) this extension before merging this pull request?