Add tangent input to glTF PBR

[pablode]

This PR adds a tangent input to the glTF PBR, similar to how it's done in the standard surface.

[pablode]

Some background info: this allows the use of tangents that are not provided by the runtime via the 'Tworld' geomprop or <tangent> node. For example, tangents could be read direcly from a mesh stream (or USD primvar) using a <geompropvalue> node, and connected to the proposed input for correct shading.

[jstone-lucasfilm]

@pablode This looks like a good proposal to me, and I'd have no reservations about integrating it to main. Is there a representative on the glTF side who might want to weigh in here, making sure that this approach for connecting tangent streams is aligned with their vision as well?

[proog128]

Looks good to me. As there is no anisotropy yet in glTF PBR, this change probably does not have an effect yet. However, as far as I can tell it is in line with the coming KHR_materials_anisotropy extension.