Fix vector tile classification invert

[bagnell]

No description provided.

[bagnell]

To test, add these two lines to the 3D Tiles Point Cloud Classification Sandcastle example:

viewer.scene.invertClassification = true;
viewer.scene.invertClassificationColor = new Cesium.Color(0.25, 0.25, 0.25, 1.0);

[lilleyse]

Thanks @bagnell.