Add support for KHR_texture_transform

[bagnell]

https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_texture_transform/README.md

[cesium-concierge]

Thanks for the pull request @bagnell!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[bagnell]

Here is a Sandcastle example of a textured box using the scale property of the extension to repeat the texture 3x in the t direction and 5x in the s direction:

var viewer = new Cesium.Viewer('cesiumContainer');
var entity = viewer.entities.add({
    position : Cesium.Cartesian3.fromDegrees(-123.0744619, 44.0503706, 10.0),
    model : {
        uri : '../../../Specs/Data/Models/Box-Texture-Transform/CesiumTexturedBoxTest.gltf',
        minimumPixelSize : 128,
        maximumScale : 20000
    }
});
viewer.trackedEntity = entity;

[cesium-concierge]

Thanks again for your contribution @bagnell!

No one has commented on this pull request in 30 days. Maintainers, can you review, merge or close to keep things tidy?

I'm going to re-bump this in 30 days. If you'd like me to stop, just comment with @cesium-concierge stop. If you want me to start again, just delete the comment.

[bagnell]

@lilleyse This is ready for another look.

[emackey]

Looking good. This will pass the Khronos sample model test.

[lilleyse]

The updates look good. Thanks @bagnell.

[emackey]

@cx20 Texture transform should appear in the May 1 release of Cesium 1.57.