Added _3DTILESDIFFUSE semantic

[lilleyse]

@pjcozzi
Is it fine that these be added by default?

[pjcozzi]

@lilleyse IIRC we discussed offline that this shouldn't be part of gltf-pipeline since it is 3D Tiles specific. I thought we planned to put it a 3D Tiles specific COLLADA2GLTF / obj2gltf workflow...maybe 3d-tiles-tools is the right place so we don't have to update every glTF converter?

[lilleyse]

Updated.
As discussed offline, it only adds the _3DTILESDIFFUSE semantic if optimizeForCesium is true.

[lilleyse]

Also CC: https://github.com/AnalyticalGraphicsInc/3d-tiles-tools/issues/21

[pjcozzi]

Is there a unit test for optimizeForCesium? Can we add an expectation for this new case?

Also update CHANGES.md.

[lilleyse]

Added unit tests and updated CHANGES.md.