st/nine: Fix fixed function fog support
st/nine: Fix fixed function fog support

Previous code had only a subcase of fog working right.

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler