avformat/mux_utils: Move ff_format_shift_data to new file for mux utils

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>

libavformat/internal.h

@@ -946,13 +946,6 @@ void ff_format_set_url(AVFormatContext *s, char *url);
 
 void avpriv_register_devices(const AVOutputFormat * const o[], const AVInputFormat * const i[]);
 
-/**
- * Make shift_size amount of space at read_start by shifting data in the output
- * at read_start until the current IO position. The underlying IO context must
- * be seekable.
- */
-int ff_format_shift_data(AVFormatContext *s, int64_t read_start, int shift_size);
-
 /**
  * Rescales a timestamp and the endpoints of an interval to which the temstamp
  * belongs, from a timebase `tb_in` to a timebase `tb_out`.