Refactor plotting functions to avoid series and use PathElement and Polygon directly

[apirogov]

While trying to find the bug in plotters plotters-rs/plotters#672 I noticed that the way polygons are rendered (border + fill) are not efficient (using AreaSeries etc).

Using Polygon, PathElement and possibly coordinate transforms directly could be simpler for basic cases.

For the vertex decorations, the series does seem to be useful, though.