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.
We do generate masks with values other than 0 or 255, although it is not currently from distance fields (but will be soon).  As a result, a single bit for each coordinate in the mask will not suffice since we would encode distances to the 0-255 range.  From your link, they map 192-255 as "inside" a glyph (water in our case), and 0-192 as "outside" the glyph (land).  Also keep in mind that a 256x256 mask is only used for tiles along the coast, single byte used otherwise.

We do need to generate water masks with some sort of alpha gradient as we approach the coast, because ultimately the resolution of coastline will not always match the resolution of your imagery.  As a result, the water effect should taper off as we approach coastlines.