It currently emulates the long-removed
avcodec_decode_audio4/avcodec_decode_video2 APIs, which obfuscates the
actual decoding flow. Restructure the decoding calls so that they
naturally follow the new avcodec_send_packet()/avcodec_receive_frame()
design.
This is not only significantly easier to read, but also shorter.
It is currently handled in the same loop as audio and video, but this
obscures the actual flow, because only one iteration is ever performed
for subtitles.
Also, avoid a pointless packet reference.