sub: prefer ffmpeg microdvd converter over internal one

We certainly don't want to maintain and improve the internal converter,
but we still need the internal one for Libav. (In the Libav case,
demux_subreader.c will be used to read the MicroDVD file.)

sub/dec_sub.c

@@ -48,9 +48,9 @@ static const struct sd_functions *const sd_list[] = {
     &sd_lavc,
     &sd_movtext,
     &sd_srt,
-    &sd_microdvd,
     &sd_lavf_srt,
     &sd_lavc_conv,
+    &sd_microdvd,
     NULL
 };