st/nine: Use correct PIPE_HANDLE_USAGE flag for frontbuffer copy
st/nine: Use correct PIPE_HANDLE_USAGE flag for frontbuffer copy

When taking screenshots we do a copy from the frontbuffer
to an allocated buffer (which we then copy to a ram buffer).

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler