Reads a PNG into a FrenchCurve.Raster.

{:ok, raster} = Linocut.Png.decode(File.read!("ship.png"))

Greyscale, greyscale with alpha, truecolour, truecolour with alpha and palette images are read, at every bit depth; sixteen-bit channels keep their high byte. A tRNS chunk gives palette entries and colour keys their alpha. Interlaced images are refused with {:error, :interlaced}; anything that is not a PNG with {:error, :not_png}.