libavformat/mtv: add test for audio magic

MTV files have the string "MP3" as audio magic on their
header. Always.

Signed-off-by: Reynaldo H. Verdejo Pinochet <r.verdejo@sisa.samsung.com>
lavfi/overlay: add yuv422 format mode
examples: add avio_reading.c example
examples/avio_reading: remove stray empty line
ffmpeg: tweak debug_ts logging and add more of it

Help with debugging timestamp issues.
lavf/avformat.h: fix typos
lavf: add output_ts_offset option to AVFormatContext

This option can be generally useful to set an output offset, needed when
setting an absolute index in the output.
doc/APIchanges: fix weird reindent