glsl: invariant qualifier is not valid for shader inputs

OpenGL 1.50 spec and newer, chapter 4.6.1 "The Invariant Qualifier",

    Only variables output from a shader can be candidates for invariance. This
    includes user-defined output variables and the built-in output variables.
    As only outputs can be declared as invariant, an invariant output from one
    shader stage will still match an input of a subsequent stage without the
    input being declared as invariant.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>