They are useless. Not only are they actually rarely in use; but
libavcodec doesn't even output them, as libavcodec has no such sample
formats for decoded audio.
Even if it should happen that we actually still need them (e.g. if doing
direct hardware output), there are better solutions. Swapping the sign
is a fast and lossless operation and can be done inplace, so AO actually
needing it could do this directly.
If you wonder why we keep U8 instead of S8: because libavcodec does it.
Yet another of these dozens of hwaccel changes. This time, libavcodec
provides utility functions, which initialize the vdpau decoder and map
codec profiles. So a lot of work the API user had to do falls away.
This also will give us support for high bit depth profiles, and possibly
HEVC once libavcodec supports it.
