SYNOPSIS:
    MARKGC_FROMRGB allowed_classes

DESCRIPTION:
    Calculate an approximate GC image from an RGB dataset.
    Palette is taken vom the current renderer in the current view.
    Note that the current grey value setting is not taken into account
    Conversion basically assumes a 0-511 (palette size) grey range.
    Use MARKGG_MULT/ADD to adjust to a given setup.
