add const begin/end iterators to SurfaceCollection to let it work in range-based for loops (const)
add const begin/end iterators to SurfaceCollection to let it work in range-based for loops (const)

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