* 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.
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