h264: Refactor ff_h264_decode_ref_pic_list_reordering

In preparation for MVC support.
h264: print values in case of error

Also make error style consistent and drop redundant information.
h264: store current_sps_id inside the current sps

In preparation for MVC support.
h264: drop outdated comments
h264_sei: name buffering period type consistently
h264_sei: reorder headers

Also drop unused assert.h.
Doxyfile: Only set HTML_{HEADER|FOOTER|STYLESHEET} from doxy_wrapper.sh
build: Do not pass HTML snippets and stylesheet as input to Doxygen

These files are referenced elsewhere and only generate warnings if
presented to Doxygen as input files.
doxygen: Set EXAMPLE_PATH from within doxy-wrapper.sh

This allows using the libav source path as the example path.
build: doxy: Include code examples in Doxygen documentation
doc/examples: misc Doxygen markup improvements

Add properly formatted @example tag and fix Doxygen syntax.
libx265: Fix use of uninitialized input picture

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
libx265: Remove redundant default param call

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
utvideoenc: Enable support for multiple slices and use them

The official Ut Video decoder only threads with slices, thus until
now any files encoded by the libavcodec encoder have only been
decodable with a single thread. The default slice count is now
set to subsampled_height / 120.

Also sets slices to 1 for the Ut Video encoder tests to keep them
green.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
ppc: reduce overreads when loading 8 pixels in altivec dsp functions

Altivec can only load naturally aligned vectors. To handle possibly
unaligned data a second vector is loaded from an offset of the original
location and the data is recovered through a vector permutation.
Overreads are minimal if the offset for second load points to the last
element of data. This is 7 for loading eight 8-bit pixels and overreads
are reduced from 16 bytes to 8 bytes if the pixels are 64-bit aligned.
For unaligned pixels the overread is reduced from 23 bytes to 15 bytes
in the worst case.
buffersink: document special error codes returned from av_buffersink_get_frame
lavfi doxy: add buffer{src,sink}.h to the main lavfi doxy group
h264_parser: use enum values in h264_find_frame_end()
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.