Testing spline drawing

[0HyperCube]

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

Testcases:

• Works when the viewport is zoomed/panned/tilted (Add Spline tool test for drawing with PTZ applied to the canvas view #2573)
• Works when the artboard has an offset, which #2484 recently fixed (Add Spline tool tests for drawing within a transformed artboard #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.
• Continuing to draw from a previous spline (Add tests for Spline tool to support continuing a previously drawn 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