GWTII-247: fixed npe + avoid duplicate builds for layers + default to
dpi = 72 (screenshot quality) + put new builders ahead so they are
pciked up + server upgrade (for testing print)
Merge pull request #111 from geomajas/GWTII-247-tilebased-sync-print

GWTII-247: fixed npe + avoid duplicate builds for layers + default to