Fixes issue seen with Vulkan + the quantized version of Avocado.
We already support the quantized glTF extension, which is when this
comes up. Technically this is only needed for Vulkan but gltfio does
not (and should not) know which backend is being used, so we simply
apply these transformations assuming the worst.
In practice this code usually won't be activated, and if even when it
is, it won't hurt loading time as much as other things like large
textures.
820717df0e