mplayer: don't display "-1" as chapter when chapter seek fails

Increase robustness against out of bound chapter numbers. Normally
these functions expect that the callers sanitize the chapter number.
This went wrong at least in add_seek_osd_messages() (which displayed
a chapter "-1" when chapters were not available). Make these functions
a bit friendler and add some reasonable checks and fallbacks, which
fixes the mentioned chapter seeking case as well.
osd: prevent osd bar from sticking around on seeks

This was supposed to be fixed in f897138, but there's another corner
case. Basically, set_osd_function() reset the OSD time, which is not
nice at all and breaks the logic of letting OSD elements disappear when
they're not wanted anymore. Fix this by adding a separate timer for
this.

Additionally, make sure the OSD bar is _really_ always updated when
visible. Also, redraw the OSD only if the OSD bar actually changes to
prevent redrawing too often (every vo_osd_changed() will flag that the
OSD should be redrawn, even if nothing changes).
options: parse C-style escapes for some options

Being able to insert newline characters ("\n") is useful for
--osd-status-msg, and possibly also for anything that prints to the
terminal. Espcially --term-osd-esc looks relatively useless without
being able to specify escapes.

Maybe parsing escapes should happen during command line / config parsing
instead (for all options).
Merge branch 'master' into osdbar_experiment

Conflicts:
	core/mplayer.c
undo manual "upsampling" of integer coords, use floats instead

The floats are scaled and converted to int by ass_draw_c(), so we can
avoid multiplying all coordinates by 2 manually.

Use floats for everything else too.

I hope nothing actually changes (except maybe how some things are
rounded) and that nothing broke.
place symbol a bit more far to the right