Previously the string for AV_CH_LAYOUT_5POINT{0,1}_BACK was the same
of the string for AV_CH_LAYOUT_5POINT{0,1}.
This is required for discerning the two channel layouts, given a
channel layout string.
It is pretty hopeless that other considerable projects will adopt
libavutil alone in other projects. Projects that need small footprint
are better off with more specialized libraries such as gnulib or rather
just copy the necessary parts that they need. With this in mind, nobody
is helped by having libavutil and libavcore split. In order to ease
maintenance inside and around FFmpeg and to reduce confusion where to
put common code, avcore's functionality is merged (back) to avutil.
Signed-off-by: Reinhard Tartler <siretart@tauware.de>