Incredibly inefficient, but I don't yet know how to efficiently encode
undo mutations that can span multiple lines.
There seems to be one bug related to creating new drawings; they're not
spawning events and undoing past drawing creation has some weird
artifacts. Redo seems to consistently work, though.
Manual test used here:
abc
```lines
{"p1":{"y":72,"x":82},"mode":"line","p2":{"y":29,"x":169}}
```
def
```lines
{"p1":{"y":36,"x":56},"mode":"line","p2":{"y":59,"x":163}}
```
```lines
```
ghi
jkl
Hitting page-down moves the cursor from abc to ghi. The 'ghi' line
should be fully visible on screen.