Add a material property to chose texture orientation relative to the camera viewpoint

Hello,
I want to be able to decide the orientation of a texture image applied to an entity.polygon relative to the camera viewpoint.

Now it seems totally random (or I didn't get how the texture orientation is chosen); sometime the text on an image is readable, and sometime it's flipped left<->right.

I'm currently applying the texture as follow:

entity.polygon.material = new Cesium.ImageMaterialProperty({
    image: './image.jpg',
    alpha: 0.5
});

In addition, the alpha value doesn't apply. It would be great to be able to add transparency to the texture.
Thanks.

Start with the entity.polygon material path and the Cesium.ImageMaterialProperty API shown in the issue. Trace how texture orientation and alpha are passed to polygon rendering, then identify existing rendering tests or examples for material properties. Done means a material option can control camera-relative texture orientation and alpha transparency works as documented.