`color::swap` - add missing `premultiplied_` (ref #4137)
`color::swap` - add missing `premultiplied_` (ref #4137)

mapnik/mapnik,naturalatlas/mapnik,tomhughes/mapnik,tomhughes/mapnik,mapnik/mapnik,naturalatlas/mapnik,lightmare/mapnik,mapnik/mapnik,tomhughes/mapnik,naturalatlas/mapnik,lightmare/mapnik,lightmare/mapnik,tomhughes/mapnik,mapnik/mapnik,naturalatlas/mapnik,lightmare/mapnik