glsl: add varyings to resource list only with SSO

glsl: add varyings to resource list only with SSO

Varyings can be considered inputs or outputs of a program only when
SSO is in use. With multi-stage programs, inputs contain only inputs
for first stage and outputs contains outputs of the final shader stage.

I've tested that fix works for Assault Android Cactus (demo version)
and does not cause Piglit or CTS regressions in glGetProgramiv tests.

Following ES 3.1 CTS separate shader tests that do query properties
of varyings in SSO shader programs pass:

   ES31-CTS.program_interface_query.separate-programs-vertex
   ES31-CTS.program_interface_query.separate-programs-fragment

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92122