st/nine: Fix FillColor Flag check
st/nine: Fix FillColor Flag check

IT is better check if the surface was created with RT flag,
instead of checking capability (llvmpipe was complaining)

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler