#38 Unpaint feature

[Willie Sippel]

A feature like the unpaint in Newtek Aura would be great:

You do a little drawing, and as long as you don't
switch the tool or color, if you hold down spacebar,
you can use your selected tool to undo the last drawing
by simply painting over it. It's a lot faster and more
intuitive as undo, 'cause you are able to 'undo' half
of the third-last line without undoing the rest or
erasing another line on the same layer, if this line
was drawn using another tool or color.