dvdnav: fix incorrect clipping of highlights
player: don't store subtitle renderer in osd_state

This doesn't have much value. It can't be accessed by anything else than
the actual subtitle renderer (sd_ass.c). sd_ass.c could create the
renderer itself, except that we apparently want to save memory (and some
font loading time) when using ordered chapters or multiple subtitle
tracks.
osd: use separate ASS_Renderer for each OSD object

This simplifies things, although it is slightly less efficient (probably
uses a bit more memory).

This also happens to fix that the OSC dropped the libass cache on every
frame.
osd_libass: update styles when OSD changes PlayRes

The OSD style settings depend on the PlayRes, simply because all style
values are implicitly scaled by the PlayResY in libass. Also, the OSC
changes the PlayResY in certain situations, so something could go wrong.
But not sure if this actually matters in practice.
player: don't temporarily disable terminal handling when reloading files

There's no reason why we should do this. For some reason, the existing
code reset terminal handling to default after unloading a file, just to
initialize it again when loading a new file.

Might be related to github issue #412, although I don't think it helps,
since the default SIGTERM handler _should_ kill the mpv process.
(It's still a nice simplification, though.)