src/img2pdf.py: do not use PNG encoding for CMYK input

src/img2pdf.py

@@ -830,6 +830,13 @@ def read_images(rawdata, colorspace, first_frame_only=False):
             newimg = imgdata
         else:
             raise ValueError("unknown or unsupported colorspace: %s" % color.name)
+        # the PNG format does not support CMYK, so we fall back to normal
+        # compression
+        if color in [Colorspace.CMYK, Colorspace["CMYK;I"]]:
+            imggz = zlib.compress(newimg.tobytes())
+            result.append((color, ndpi, imgformat, imggz, imgwidthpx,
+                           imgheightpx, []))
+        else:
             # cheapo version to retrieve a PNG encoding of the payload is to
             # just save it with PIL. In the future this could be replaced by
             # dedicated function applying the Paeth PNG filter to the raw pixel