collada2gltf: it seems better not to add #version to glTF shader files

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
libgltf: Append shader language version to the shader files

In general glTF shader files does not contain version
directives and in some case it make shader compiler
using GLSL 1.1 which leads to that the shader compiler
fails.
So we need to append the choosen version number which is
GLSL 1.3 in case of libgltf, but this also means that
from that point OpenGL 3.0 is the new reuirements since
GLSL 1.3 is available only from that version.

Change-Id: Ic4382266432ea474aeb3e603b32a998b9aeed280