always allow to cancel eraser mode

always allow to cancel eraser mode

Fix for https://gna.org/bugs/index.php?18616

And a similar problem fixed by this commit:
<liquidApe> in trunk - draw some, press 'e' to go into eraser mode, then
flip pen over and erase some, then flip pen back over, it will flip back
into the 'e' eraser. Press 'e' again to toggle out of eraser mode and
note you can't get out of eraser mode