* Return a negative value on error, otherwise return the number of bytes used.
* If no subtitle could be decompressed, got_sub_ptr is zero.
* Otherwise, the subtitle is stored in *sub.
+ * Note that CODEC_CAP_DR1 is not available for subtitle codecs. This is for
+ * simplicity, because the performance difference is expect to be negligible
+ * and reusing a get_buffer written for video codecs would probably perform badly
+ * due to a potentially very different allocation pattern.
*
* @param avctx the codec context
* @param[out] sub The AVSubtitle in which the decoded subtitle will be stored.