Implementer draw_field_line

// Compute and then draw all the points belonging to a field line, // starting from pos0. // Returns false if pos0 is not a valid position // (for example if pos0 is too close to a charge). static bool draw_field_line(struct gfx_context_t *ctxt, charge_t *charges, int num_charges, double dx, vec2 pos0, double x0,double x1, double y0, double y1);