st/nine: Introduce helper for Position shader input
st/nine: Introduce helper for Position shader input

Cc: "11.2" <mesa-stable@lists.freedesktop.org>

Signed-off-by: Axel Davy <2b6ef604b998fb81662f85c329e4fc020eae8e5d@ens.fr>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler