demux: simplify stream ID business, fix issue with cover art

The stream ID handling as it was changed in commit 654c34f was still
a little bit insane, and caused a regression with the cover art hack
(the stream set in demux->video->sh was incorrect for demux_lavf).

Simplify by always using stream_index for demux_stream->id, and getting
rid of that tid thing. It turns out that the id for subtitles isn't
special either (maybe demux_ts.c was the only thing left that required
this).
subreader: fix out of bound write access when parsing .srt

This broke .srt subtitles on gcc-4.8. The breakage was relatively
subtle: it set all hour components to 0, while everything else was
parsed successfully.

But the problem is really that sscanf wrote 1 byte past the sep
variable (or more, for invalid/specially prepared input). The %[..]
format specifier is unbounded. Fix that by letting sscanf drop the
parsed contents with "*", and also make it skip only one input
character by adding "1" (=> "%*1[...").

The out of bound write could easily lead to security issues.

Also, this change makes .srt subtitle parsing slightly more strict.
Strictly speaking this is an unrelated change, but do it anyway. It's
more correct.