glsl/linker: Check the invariance of built-in special variables

glsl/linker: Check the invariance of built-in special variables

From Section 4.6.4 (Invariance and Linkage) of the GLSL ES 1.0 specification

    "The invariance of varyings that are declared in both the vertex and
     fragment shaders must match. For the built-in special variables,
     gl_FragCoord can only be declared invariant if and only if
     gl_Position is declared invariant. Similarly gl_PointCoord can only
     be declared invariant if and only if gl_PointSize is declared
     invariant. It is an error to declare gl_FrontFacing as invariant.
     The invariance of gl_FrontFacing is the same as the invariance of
     gl_Position."

Fixes:
    * glsl-pcoord-invariant.shader_test
    * glsl-fcoord-invariant.shader_test
    * glsl-fface-invariant.shader_test

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107734
Signed-off-by: Vadym Shovkoplias <vadym.shovkoplias@globallogic.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>