Update gnuplot to version 6.0.3 / rev 100 via SR 1284714
    
    https://build.opensuse.org/request/show/1284714
    by user WernerFink + anag_factory
    - Add patch gnuplot-6.0.3-backward_compat.patch
    
    - Update to gnuplot 6.0.3
      * NEW backport "contourfill {at base} {fs {no}border}"
      * NEW backport "save changes <filename>"
            This variant saves only the differences between the current
            state and the state at the start of the session.
      * NEW backport "plot <data> ... if (filter_condition)"
            Input lines that satisfy the if condition are processed as usual.
            Lines that fail are essentially ignored.
      * NEW backport watchpoint label improvements from 6.1
            Each watch target can have its own label, generated by a user function
      * CHANGE 3D polygon objects can have per-object fill border properties.
            The restriction that all 3D polygons share a single set of
            properties from "set pm3d" remains t