Disallow input images with alpha channel as it cannot be preserved in the PDF and would thus make img2pdf not lossless

src/img2pdf.py

@@ -637,6 +637,9 @@ def get_imgmetadata(imgdata, imgformat, default_dpi, colorspace, rawdata=None):
         ndpi = (int(round(ndpi[0])), int(round(ndpi[1])))
         ics = imgdata.mode
 
+    if ics in ["LA", "PA", "RGBA"]:
+        raise Exception("Image contains transparency which cannot be retained in PDF")
+
     # Since commit 07a96209597c5e8dfe785c757d7051ce67a980fb or release 4.1.0
     # Pillow retrieves the DPI from EXIF if it cannot find the DPI in the JPEG
     # header. In that case it can happen that the horizontal and vertical DPI