glTF exporter: do not bake selected skinned meshes, as glTF transforms must be ignored on skinned meshes

glTF exporter: performance: cache parent/children data to avoid bad .children performance when used a lot
Thanks rotoglup!

Fix when animation are not baked

    KHR_materials_ior
    KHR_materials_sheen
    KHR_materials_specular
    KHR_materials_transmission
    KHR_materials_variants
    KHR_materials_emissive_strength
    KHR_materials_volume

Documentation update is still in progress

Contributed by luzpaz.

Differential Revision: https://developer.blender.org/D15328