audio_mix: fix channel order in mix_1_to_2_fltp_flt_c

CC:libav-stable@libav.org
FATE: add lavr mixing tests
FATE: add lavr resampling tests
lavc doxy: remove false statements about alignment requirements.

The packet data does not need to be aligned.
lavc doxy: extend/clarify avcodec_decode_audio4() doxy

Elaborate on flushing the decoder.