LAYER_VIAS needs to get set for them to print at all, and GetLOD
needs too look at the view layer visibiilties, not the board layer
visibilities.
Also implements a more pad-like strategy for printing blind/buried
vias when printing one layer per page (where the top/bottom arcs
don't make sense).
Fixes https://gitlab.com/kicad/code/kicad/issues/1938
(cherry picked from commit dce42612d3)
Jeff Young
Maciej Suminski