This is useful for paletted data where the rastercolorizer is overkill and just one color needs to be marked transparent. It can be accomplished by using gdal_translate with -a_nodata <value> but we should allow this to be done in mapnik to avoid needing to pre-process the data for this simple case.
support nodata for paletted images and allow user to set nodata on-th…
…e-fly - closes #1160 and #1161 - refs #688 and refs #730 and refs #50 and refs #1018
solved by 46b16c9, closing.