Khan/khan-exercises — pull request #68397 by theinternetftw (2013-08-30 11:07:46+00:00)
Title: Fix scratchpad undo

Three undo bugs fixed:
1) Undo didn't work at all (mousedown was saving state when
toolbar was clicked).
2) Undo was adding back in clipped lines.
3) Using the eraser added state to undo even if you didn't
actually erase anything. (N.B. stopped using shapes.length
here as it doesn't work: length doesn't change when things
are .remove()'d)