movenc: ilbc needs audio_vbr set.

Without this the block_align or bitrate value is not available to the decoder

Fixes Ticket2858

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 3d64845600c6486a2706b118a81805f3bf4d3db5)
swr: clean layouts before checking sanity

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 6dfffe92004dfd8c79d18791f28a2b1c7e387845)
swr/rematrix: Fix handling of AV_CH_LAYOUT_STEREO_DOWNMIX output

Fixes Ticket2859

Note, testcases related to the downmix channels are welcome.
(id like to make sure this is working correctly now, as obviously it didnt
 work before ...)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit c56d4dab039b352961cca298d753b04e2f2fd990)