Testing spline drawing

[0HyperCube]

Further work on this shouldn't occur until #2368 is merged.

Testcases:

• [x] Works when the viewport is zoomed/panned/tilted (#2573)
• [ ] Works when the artboard has an offset, which #2484 recently fixed (#2575)
• [ ] 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.
• [x] Continuing to draw from a previous spline (#2591)
• [ ] Continuing drawing when the other spline is transformed and in a group (currently broken, see #2482)
• [ ] Continuing to draw from some other path