ac3: update AC3PreferredStereoDownmixMode.

'11b' is reserved in the A/52 specification,
but newer encoders use it to indicate a Dolby
Pro Logic II compatible Lt/Rt downmix.
lavu: add AV_FRAME_DATA_DOWNMIX_INFO side data type.

Can be obtained and/or added via
av_downmix_info_get_side_data().
ac3dec: set AV_FRAME_DATA_DOWNMIX_INFO side data.
NOT FOR COMMIT

Testing code.