Perimeter loop edition [beta]

new features

• perimeters can be merged into a continous loop (option in perimeter/advanced tab)
• new option for dense_infill algorithm : auto (no turn into the sparse infill) vs Anchored (use the Bridge anchor parameter to offset the dense layer a bit)

bugfixes

• support generation crash (on big models)
• thin_wall (& gap fill) lines that go to 0,0