Comment 3 for bug 699454

add --export-all-layers:
(merged with changes from tracker patch 1833934
submitted by Bert Timmerman lambert63;
feature request 1830064 filed by David Griffith)
export all layers including the empty ones

Changes from the original patch:
(1) empty drills are exported, too
(2) assembly and "invisible" layers are not exported
(3) an (unneeded) aperture added to empty Gerbers so
    gerbv wouldn't tell they are RS-274D.