udev-device-plug-handler: Scan to PNG

Instead of scanning to the default PNM format, we scan to PNG. PNM isn't
that well-supported in normal image handling software.

udev-device-plug-handler/udev-device-plug-handler

@@ -134,9 +134,10 @@ class CanonLIDE400(KnownDevice):
             '-y', '297mm',
             '--mode=Color',
             '--resolution', '300dpi',
+            '--format', 'png',
             '--button-controlled=yes',
             # '-p',  # progress
-            f"--batch=scan_{now.strftime('%H%M%S')}_%04d.pnm"
+            f"--batch=scan_{now.strftime('%H%M%S')}_%04d.png"
         ]
         CanonLIDE400.__process = subprocess.Popen(cmd, stdout=subprocess.DEVNULL, cwd=directory)
         print("Handled Canon LiDE 400")