Extension to support texture transformation

[jackcaron]

It's quite common to defined at the texture level (or in glTF case, the sampler) how we want that image to be tiled, have an offset, or even a rotation.

Defining a new extension, that could be added inside the sampler (that's where it makes the most sense) on how to transform the UVs before reading the associated textures.

And I think it makes sense as an extension because this would require modifying and compiling the fragment shader differently, depending on each texture access. But Three.js already supports that.

[jackcaron]

Duplicated of #1335