support applying offsets for relative positioning of the mask image
support applying offsets for relative positioning of the mask image

tomhughes/node-mapnik,CartoDB/node-mapnik,langateam/node-mapnik,stefanklug/node-mapnik,mapnik/node-mapnik,CartoDB/node-mapnik,langateam/node-mapnik,tomhughes/node-mapnik,Uli1/node-mapnik,CartoDB/node-mapnik,stefanklug/node-mapnik,tomhughes/node-mapnik,CartoDB/node-mapnik,stefanklug/node-mapnik,langateam/node-mapnik,tomhughes/node-mapnik,MaxSem/node-mapnik,mapnik/node-mapnik,mojodna/node-mapnik,langateam/node-mapnik,MaxSem/node-mapnik,mojodna/node-mapnik,Uli1/node-mapnik,gravitystorm/node-mapnik,gravitystorm/node-mapnik,stefanklug/node-mapnik,CartoDB/node-mapnik,langateam/node-mapnik,gravitystorm/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,Uli1/node-mapnik,mojodna/node-mapnik,mojodna/node-mapnik,mapnik/node-mapnik,gravitystorm/node-mapnik,MaxSem/node-mapnik,Uli1/node-mapnik,mapnik/node-mapnik,MaxSem/node-mapnik