USDZExporter: Use object names

[mkeblx]

Use an object's name (when present) in the output.

To conform to USD, ensure that name's are valid and unique.

---

Before:

        def Xform "Object_184" (
              prepend apiSchemas = ["MaterialBindingAPI"]
              prepend references = @./geometries/Geometry_191.usda@</Geometry>
          )
          {
              rel material:binding = </Materials/Material_103>
              uniform token[] xformOpOrder = ["xformOp:transform"]
          }

          def Xform "Object_189" (
              prepend apiSchemas = ["MaterialBindingAPI"]
              prepend references = @./geometries/Geometry_192.usda@</Geometry>
          )
          {
              rel material:binding = </Materials/Material_104>
              uniform token[] xformOpOrder = ["xformOp:transform"]
          }

After:

            def Xform "Sphere" (
                prepend apiSchemas = ["MaterialBindingAPI"]
                prepend references = @./geometries/Geometry_192.usda@</Geometry>
            )
            {
                rel material:binding = </Materials/Material_104>
                uniform token[] xformOpOrder = ["xformOp:transform"]
            }

            def Xform "Cylinder" (
                prepend apiSchemas = ["MaterialBindingAPI"]
                prepend references = @./geometries/Geometry_193.usda@</Geometry>
            )
            {
                rel material:binding = </Materials/Material_105>
)
                uniform token[] xformOpOrder = ["xformOp:transform"]
            }

            def Xform "Sphere_191" (
                prepend apiSchemas = ["MaterialBindingAPI"]
                prepend references = @./geometries/Geometry_192.usda@</Geometry>
            )
            {
                rel material:binding = </Materials/Material_104>
                uniform token[] xformOpOrder = ["xformOp:transform"]
            }