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

未指定文件、测试或入口点。首先在绘图工具中重现尚未检查的样条线案例，使用 issues #2482 和 #2484 作为所述的变换上下文；完成意味着剩余的检查清单案例都能正常工作，包括变换后的路径和分组路径、在起点闭合，以及从另一条路径继续。

由索引模型根据 Issue 内容生成。