glsl: invariant qualifier is not valid for shader inputs in GLSL ES 3.00

glsl: invariant qualifier is not valid for shader inputs in GLSL ES 3.00

GLSL ES 3.00 spec, 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.

This patch fixes the following dEQP tests:

dEQP-GLES3.functional.shaders.qualification_order.variables.valid.invariant_interp_storage_precision
dEQP-GLES3.functional.shaders.qualification_order.variables.valid.invariant_interp_storage
dEQP-GLES3.functional.shaders.qualification_order.variables.valid.invariant_storage_precision
dEQP-GLES3.functional.shaders.qualification_order.variables.valid.invariant_storage
dEQP-GLES3.functional.shaders.qualification_order.variables.invalid.invariant_interp_storage_precision_invariant_input
dEQP-GLES3.functional.shaders.qualification_order.variables.invalid.invariant_interp_storage_invariant_input
dEQP-GLES3.functional.shaders.qualification_order.variables.invalid.invariant_storage_precision_invariant_input
dEQP-GLES3.functional.shaders.qualification_order.variables.invalid.invariant_storage_invariant_input

No piglit regressions observed.

v2:
- Add spec content in the code

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>