Some quantized-mesh related improvements

[kring]

From the changelog:

• Added LayerWriter to the CesiumQuantizedMeshTerrain library and namespace.
• The glTF accessor for the texture coordinates created by RasterOverlayUtilities::createRasterOverlayTextureCoordinates now have min/max values that accurately reflect the range of values. Previously, the minimum was always set to 0.0 and the maximum to 1.0.

Also:

• Removed an unused struct from QuantizedMeshLoader.cpp.
• Fixed incorrect doc comment syntax in GlobeRectangle.h.