Load landmass data from tiles

Typical usage:
marble-vectorosm-tilecreator -o <dir> -z 11,13,15 -m landmass/7 map.o5m

Where landmass/7 is a directory created before using e.g.
marble-vectorosm-tilecreator -o landmass -z 7 -k landmass.shp
(note -o landmass and -z 7 resulting in landmass/7/... tiles)