plotter: consider modifying Contour to produce reproducible plots

as noted in https://github.com/gonum/plot/pull/620, plots produced by `Contour` are not reproducible.
they look visually the same but the vg operations underneath are different.
this leads to plots not being exactly the same at the binary level (except for `pdf`. probably a feature of the backend)

the "naive" draw strategy does generate reproducible plots.