Document how tangents and bi-tangents are treated

[Moguri]

glTF uses a 4-component tangent vector and does not supply a bi-tangent (this is assumed to be calculated in the shader). As such, models loaded by panda3d-gltf will follow this convention. There is also talk about following this in Panda as well. However, at the moment, this means that models loaded by this project will not work correctly with the normal mapping done by the auto-shader (simplepbr does support these tangents).

[rdb]

A further note is that the shader generator is currently unable to process the tangents that panda3d-gltf uses, because it specifically looks for a tangent.0 column (with 0 being the name of the texture coordinate set that panda3d-gltf generates), as opposed to a generic tangent column. I am not yet sure whether this is something that should be changed in simplepbr, panda3d-gltf, or panda3d itself.