Canvas missing update when undoing strokes created with polyline tool.

[Jose-Moreno]

Issue Summary

When drawing any given line with the polyline tool, undoing the stroke will result in the buffer not being cleaned / updated from the stroke until the polyline tool is used to draw again.

The after effects of the buffer not being updated (or the stroke "event" not being added?) hold remarkable resemblance to the problem from #1196 and fixing this could potentially help fixing that one which seems to be more elusive as you have to jerk your stylus in a certain way while the stroke is being computed, in order to reproduce.

Expected Results

Creating any kind of line with any drawing tool should never become fixated to the canvas as this can and will cause confusion in the end user.

Video or Image Reference

https://youtu.be/onnZfHpHNWw

Steps to reproduce

1. Create a new file
2. Pick the polyline tool and draw a single "complete" stroke (that is, click, move the mouse and double click to finalize the stroke drawing mode)
3. Undo
4. Notice that the stroke is still present
5. Using the polyline tool draw a new stroke away from the previous one click on the canvas once without moving the mouse. Nothing happens. Now move the mouse slightly and you'll notice that the previous stroke disappears as soon as move the pointer.

System Information

• Pencil2D Version:

Version: 0.6.5

commit: 50a0c7c
date: 2020-08-01_01:32:15

Operating System: Windows 10 (10.0)
CPU Architecture: x86_64
Qt Version: 5.12.9