Revert "libgltf: download from libgltf subfolder"

This reverts commit ca13d1f6988bc1671000a30af483b00eb15bcdb8.
Revert "Update libgltf: various fixes, memory usage optimization"

This reverts commit fc40081993d29c422fd19bb80f4d9db06b4dce0d.

Some files are missing from the new tarball, so revert
this until this is fixed.