glsl: allow layout qualifier overrides with ARB_shading_language_420pack
glsl: allow layout qualifier overrides with ARB_shading_language_420pack

GLSL 4.20 allows overriding the layout qualifiers.

This helps fix:
GL45-CTS.shading_language_420pack.qualifier_override_layout

Reviewed-by: Timothy Arceri <5dd83789ef3714029244367ba1ca09f1d7627408@collabora.com>
Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler