encode: deal with codec->time_base deprecation

This seems to work with both Libav 10 and FFmpeg d3e51b41.
sub: add detection via BOM

Useful for Windows stuff. Actually, ENCA support should catch this, but,
well, whatever, everyone seems to hate ENCA.

Detection with BOM is trivial, although it needs some hackery to
integrate it with the existing autodetection support. For one, change
the default value of --sub-codepage to make this easier.

Probably fixes issue #937 (the second part).
sub: offset subtitle timing to video start PTS

This allows using external subtitle files with e.g. transport stream
files that don't start at time 0.

Note that if the .ts file has timestamp resets, everything goes south.
But I guess this was already the case before, unless there are external
subtitle files that also include timestamp resets, which is unlikely.
(On the other hand, you could for example expect that it works with
embedded DVB subtitles, that were somehow captured from the same stream
and use the same timestamps.)
player: remove something DVD specific

This is not needed anymore, because demux_disc isolates us from this
crap.