They are used for chamfered round rect pads, and can be used for custom shaped pads.
No actual change currently, but the shape rotation of custom pads and chamfered rr pads
can be now used in gerber plots.
plotter.h contains now only a header common to all plotters.
The code does not actually change, but it allows modifying a specific plotter
without recompiling most of kicad files.