The Raster type represent images/pixmaps, used in conjunction with
the raster :: Raster -> Picture constructor:
rasterName :: String -> Raster mkPixmapRaster :: Size -> Pixmap -> Raster mkBitmapRaster :: Size -> Bitmap -> Raster mkImageRaster :: Size -> Image -> Raster mkRaster :: Array Size Colour -> Raster mkMutRaster :: _MutableArray _RealWorld Size Colour -> Raster getRasterSize :: Raster -> Size