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.
ac3dec: set AV_FRAME_DATA_DOWNMIX_INFO side data.
Prepare for 10_beta1 Release
g2meet: validate bpp and bitmasks in the display info

Signed-off-by: Kostya Shishkov <kostya.shishkov@gmail.com>
Move all example programs to doc/examples

Also drop support for building examples in library directories.
doc: Fix project name typo
h264: use avpriv_request_sample for chroma_format_idc