use 'overlayadd/remove' instead of 'layeradd/remove' events. in theory should be more efficient as layeradd/remove gets called for every single portal/link/field/etc on the map
create missing link data from portal link info - so short links not sent are shown in the map

note: need to add code to hide short links from the map - and update the shown/hidden links after a zoom
(similar to the portal cluster showing/hiding portals after a zoom)
only add link entities to the layer if they're long enough to be visible
an improvement on 4838de6788ad27bbf6357e1ac9099744483a9e01 to prevent excessive links being drawn when lots of portals are loaded