sub: handle vobsub-in-mp4

The mplayer decoder (spudec.c) actually handled this. There was explicit
code for binary palettes (16 32 bit values), and the subtitle resolution
was handled by video resolution coincidentally matching the subtitle
resolution.

Whoever puts vobsub into mp4 should be punished.

Fixes the sample gundam_sample.mp4, closes github issue #547.

sub/sd.h

@@ -25,6 +25,9 @@ struct sd {
     // the resolution of the VO, nor does it have to be the OSD resolution.
     int sub_video_w, sub_video_h;
 
+    // Resolution hints stored in mp4 files.
+    int sub_stream_w, sub_stream_h;
+
     // Make sd_ass use an existing track
     struct ass_track *ass_track;