According to its documentation it returns "pts of the last muxed packet
+ its duration", but the value it actually returns right now is
(possibly guessed) dts after muxer-internal bitstream filtering (if
any).
This function was added for ffmpeg.c, but it is not used there anymore.
Since the value it returns is ill-defined and so inappropriate for any
serious use, deprecate it.
It is only used by muxers. Given that it is not part of
the core muxing code and given that mux.c is already big enough,
it is moved to a new file for utility functions for muxing.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
