v.to.rast: Add points without segfault with densification flag (-d)

[HuidaeCho]

trafficstars

This PR addresses https://github.com/OSGeo/grass/issues/3105. It implements a new plot_point() function that uses the internal state in dense_line.c. Calling G_plot_point() with -d flag causes a segfault because the static state in libgis is not shared with any modules.