libgltf: check for #version string in the shader before add an own one.

Change-Id: I69dbbabc5774cbce7943e77f41bb42ed7a118bcf
(cherry picked from commit 6be202585f201deaf07c57e0f2cf722cce91165e)
Reviewed-on: https://gerrit.libreoffice.org/10957
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>