Here only handle precision. Using precision unconditionally
causing problems when shader source is compiled with GLSL 1.2.
The #version dircetives can be added by the glTF parser.
For example libgltf implementation:
https://gerrit.libreoffice.org/gitweb?p=libgltf.git;a=commit;
h=e4544a8b5ca8470a96bf28c6ccdb7461e78ca293

Change-Id: I934cdfa8651a8787fbcd476cea9892bf7e89dd11