Testing spline drawing

Test cases:
- [x] Works when the viewport is zoomed/panned/tilted (#2573)
- [x] Works when the artboard has an offset, which [#2484](https://github.com/GraphiteEditor/Graphite/issues/2484) recently fixed (#2575)
- [x] Continuing to draw from a previous spline (#2591)
- [ ] Works when the artboard has a transform before it
- [ ] Closed splines can be drawn by clicking on the start. The tool should stop drawing. The second derivative of the curves should be continuous.
- [ ] Continuing drawing when the other spline is transformed and in a group (currently broken, see [#2482](https://github.com/GraphiteEditor/Graphite/issues/2482))
- [ ] Continuing to draw from some other path