Same as add_curve_to except the coordinates are relative to the previous node.
| this |
a Path |
| x_1 |
the x coordinate of the first control point |
| y_1 |
the y coordinate of the first control point |
| x_2 |
the x coordinate of the second control point |
| y_2 |
the y coordinate of the second control point |
| x_3 |
the x coordinate of the third control point |
| y_3 |
the y coordinate of the third control point |