add avcodec_get_subtitle_defaults() to initialize AVSubtitle struct

Call this new function before decode() to replace the custom and
inconsistant initialization in various decoders.
This function is equivalent to avcodec_get_frame_defaults() for AVFrame.

Signed-off-by: Aurelien Jacobs <aurel@gnuage.org>

libavcodec/dvdsubdec.c

@@ -173,7 +173,6 @@ static int decode_dvd_subtitles(AVSubtitle *sub_header,
 
     if (buf_size < 10)
         return -1;
-    memset(sub_header, 0, sizeof(*sub_header));
 
     if (AV_RB16(buf) == 0) {   /* HD subpicture with 4-byte offsets */
         big_offsets = 1;