glsl: serialize data from glTransformFeedbackVaryings

While XFB has been enabled for cache, we did not serialize enough
data for the whole API to work (such as glGetProgramiv).

Fixes: 6d830940 "Allow shader cache usage with transform feedback"
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106907


Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
(cherry picked from commit ab2643e4)