Add an audio transcoding example.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  Add an audio transcoding example.

Conflicts:
	configure
	doc/Makefile

Merged-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples/transcode_aac: fix project name

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
get_audio_buffer: fix usage where channels are not set but layout is

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples/transcode_aac: switch to swresample

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples/transcode_aac: remove non converted codepath

This codepath is not implemented and just crashes, also its simpler
without special cases, which makes sense for an example

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>