Undo doesn't work if frame changes during drawing operation.

[bcampbell]

Steps To Replicate:

1. create an image with a few frames
2. hold down "2" key to cycle through frames
3. hold down left mouse button to draw and spend a few seconds drawing on the frames as they flip by.

Observed: only the last frame can be undone. All the others are left with drawing still on them.

Expected: Should be able to undo all the drawing on all the frames.