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.
Sounds good.  Let's keep it then.  Distance fields would still have to be a separate extension even if they use the same data layout since they will need different code to render.