avstring: Mark some character handling functions av_const

Signed-off-by: Martin Storsjö <martin@martin.st>
lavf: Add an option for avoiding negative timestamps

This is the same logic as is invoked on AVFMT_TS_NEGATIVE,
but which can be enabled manually, or can be enabled
in muxers which only need it in certain conditions.

Also allow using the same mechanism to force streams to start
at 0.

Signed-off-by: Martin Storsjö <martin@martin.st>
lavf: Print a warning if failed to avoid negative timestamps when requested

Signed-off-by: Martin Storsjö <martin@martin.st>
Merge commit '4981baf9b803f3c4866b2e97fdadb008c62dc7ad'

* commit '4981baf9b803f3c4866b2e97fdadb008c62dc7ad':
  avstring: Mark some character handling functions av_const

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '1384df641994bf3d6cb51084290aa94752737bae'

* commit '1384df641994bf3d6cb51084290aa94752737bae':
  lavf: Add an option for avoiding negative timestamps

Conflicts:
	libavformat/avformat.h
	libavformat/mux.c
	libavformat/options_table.h
	libavformat/version.h

See: 3ba0dab76ac32f0c9f50c916a73bc64e43c1fdf9
See: a89c01253190b9eb9de8e28a3252423bf7732511
Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '897d5c3a4296f3da80b8699d1487328ca2de8e55'

* commit '897d5c3a4296f3da80b8699d1487328ca2de8e55':
  lavf: Print a warning if failed to avoid negative timestamps when requested

Conflicts:
	libavformat/mux.c

See: ec6a5fc6cca22e4eb0a91cc8fba786d41ade2032
Merged-by: Michael Niedermayer <michaelni@gmx.at>