glsl: Make invariant outputs in ES fragment shader not to cause error

glsl: Make invariant outputs in ES fragment shader not to cause error

In all GLSL ES versions output variables in fragment shader are allowed
to be invariant.

 From Section 4.6.1 ("The Invariant Qualifier") GLSL ES 1.00 spec:
 "Only the following variables may be declared as invariant:
   ...
   - Built-in special variables output from the fragment shader."

 From Section 4.6.1 ("The Invariant Qualifier") GLSL ES 3.00 spec:
 "Only variables output from a shader can be candidates for invariance."

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107842