When drawing with the locked canvas, if a brush stroke gets too close to the edge of the canvas and its stroke would extend past the edge of the canvas, the edge of that stroke will be seen after unlocking the canvas.
This could cause a problem if you unlock the canvas and now have a bunch of strokes just outside the init image area, which are far back in undo history and you cannot easily erase.
With this change, lines drawn while the canvas is locked get clipped to the initial image bbox, fixing this issue.
Additionally, the merge and save to gallery functions have been updated to respect the initial image bbox so they function how you'd expect.
