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.
I agree to remove the section; I would also add that in most places, we start the description of readonly properties with `Gets` and read/write properties with `Gets or sets`.  This way you can tell just by the description (in addition to having the `@readonly` tag)