obj2sdf, add option to merge shapes with same materials, add support for transparent objects

tiny_obj_loader, load the "d" and "Tr" transparant tag

examples/ThirdPartyLibs/Wavefront/tiny_obj_loader.h

@@ -22,6 +22,7 @@ typedef struct
     float transmittance[3];
     float emission[3];
     float shininess;
+	float transparency;
 
     std::string ambient_texname;
     std::string diffuse_texname;