Support new raster colorizer modes linear-rgba and linear-bgra

[winsento]

In this PR mapnik/mapnik#4058 was added a new raster-colorizer-default-mode:

• linear-rgba
• linear-bgra

[nebulon42]

For support we need mapnik-reference to support it and a release of it. This would require a new Mapnik API version I guess. As soon as the updated mapnik-reference is used by carto it will support it then.