Fix typo in drag_start_pos assignment.
Fix typo in drag_start_pos assignment.

curieos/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,pieis2pi/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,lordofhyphens/Slic3r,pieis2pi/Slic3r,curieos/Slic3r,alexrj/Slic3r,curieos/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,curieos/Slic3r,alexrj/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r