Restore billiard plotting

[saraedum]

billiard() creates a surface from a polygon. It is essentially equivalent to polygon_double(). However, their plotting used to be different.

billiard() used to plot trajectories in a single polygon (internally using one positively and one negatively oriented copy of the polygon) whereas polygon_double() drew trajectories in two positively oriented copies of the polygon.

We should restore this feature somehow. Probably we should get rid of special plotting logic but instead have a map from the polygon_double() to the surface (with boundary) formed by the single polygon. We can then plot such trajectories by plotting their image under this map.