Load 3mx/3mxb format LOD model files generated by Bentley ContextCapture into OpenSceneGraph.

• Windows 10
• Visual Studio 2017
• OpenSceneGraph-3.6.4 (jpeg is required for loading texture)

• openCTM-1.0.3
• cJsonObject-1.2

Add script to [osgPlugins/CMakeLists.txt] so that this plugin could be added to OpenSceneGraph.

# about line: 150 ADD_PLUGIN_DIRECTORY(3mx) 

Add file extension alias to [osgDB/Registry.cpp] so that both 3mx/3mxb could be loaded.

// about line: 220 Registry::Registry() {	// ...	addFileExtensionAlias("3mxb", "3mx");	// ... }