Adds a [cubic Bézier curve](https://en.
wikipedia.org/wiki/B%C3a9zier_curve) from the current point to x3, y3 with
x1, y1 and x2, y2 as the control points.
All coordinates are given relative to the current point.
This is the relative version of [method@Gsk.PathBuilder.cubic_to].
| this |
a path builder |
| x1 |
x offset of first control point |
| y1 |
y offset of first control point |
| x2 |
x offset of second control point |
| y2 |
y offset of second control point |
| x3 |
x offset of the end of the curve |
| y3 |
y offset of the end of the curve |