lavc: add MicroDVD decoder.

Based on my MicroDVD->ASS conversion code from MPlayer
(sub/subassconvert.c).

libavcodec/allcodecs.c

@@ -397,6 +397,7 @@ void avcodec_register_all(void)
     REGISTER_ENCDEC  (ASS, ass);
     REGISTER_ENCDEC  (DVBSUB, dvbsub);
     REGISTER_ENCDEC  (DVDSUB, dvdsub);
+    REGISTER_DECODER (MICRODVD, microdvd);
     REGISTER_DECODER (PGSSUB, pgssub);
     REGISTER_ENCDEC  (SRT, srt);
     REGISTER_ENCDEC  (XSUB, xsub);