Each pad group is allowed to short nets with other pads in its group.
Legacy footprints with the "net tie" keyword hack will get a single
group auto-created with all the footprint's pads in it.
DRC and the router now allow a track to collide with copper graphic items
while entering a net-tie pad as long as the closest point in the collision
is within the pad.
DRC (and the footprint checker) now check for copper items in the
footprint shorting pads which are not in the same pad group.
Fixes https://gitlab.com/kicad/code/kicad/issues/2265
Jeff Young