Fixed divide-by-zero in D_PAD::ViewGetLOD()
Push-and-shove router now handles existing blind/buried vias properly. Can't yet create new ones.
1) Pad properties dialog can trigger divide-by-zero when "0" is entered as the pad size
2) PNS router incorrectly detects collisions with blind/buried vias that don't actually intersect the active layer
3) PNS router silently converts blind/buried vias to through-board vias when shoved