Add support for Quantized-Mesh terrain tiles with more than 64k vertices

CesiumTerrainProvider, QuantizedMeshTerrainData, and related workers have been updated to handle 32bit index buffers for terrain meshes.  We see these types of dense meshes when noisy high resolution data is processed, or high resolution data containing features not well suited for mesh simplification (e.g., extruded building footprints baked into the input terrain data).

I've also tweaked the coefficients for ambient and diffuse lighting to brighten the globe when vertex lighting is enabled. cc @pjcozzi 
Just throw an exception like we did before.  Most users will have this extension (not sure about mobile, but I expect most of them have it).  We can add a backup if there is enough need.