jpegdec: By default assume that we're working on non-packetized input

Only detecting this in set_format() does not work because we might
not get any caps at all, e.g. from filesrc.

ext/jpeg/gstjpegdec.c

@@ -1406,6 +1406,8 @@ gst_jpeg_dec_start (GstVideoDecoder * bdec)
   dec->parse_entropy_len = 0;
   dec->parse_resync = FALSE;
 
+  gst_video_decoder_set_packetized (dec, FALSE);
+
   return TRUE;
 }