stream/cache: handle failure of seeking underlying stream

This could for example happen when serving an incomplete file from http,
and the demuxer tries reading data from the end of the file when opening
it (e.g. with avi). Seeking past EOF fails with http, so the file could
never be opened, and the cache would get stuck trying to seek to the
position.

We can't really make the cache report seek failure directly (it would
suck for various reasons), so just make the cache report EOF if seeking
fails.
sub: add --ass-style-override=force option

(The old "force" choice of that option is renamed to "force-default".)

This allows overriding native ASS script subtitle styles with the style
provided by the --sub-text-* options (like --sub-text-font etc.). This
is disabled by default, and needs to be explicitly enabled with the
--ass-style-override=force option and input property.

This uses in fact exactly the same options (--sub-text-*) and semantics
as the ones used to configure unstyled text subtitles.

It's recommended to combine this with this in the mpv config file:

ass-force-style="ScaledBorderAndShadow=1"   # work around dumb libass behavior

Also, adding a key binding to toggle this behavior should be added,
because overriding can easily break:

L cycle ass-style-override

This would cycle override behavior on Shift+L and allows quickly
disabling/enabling style overrides.

Note: ASS should be considered a vector format rather than a subtitle
format. There is no easy or reliable way to determine whether the style
of a given subtitle event can be changed without destroying visuals or
not. This patch relies on a simple heuristic, which often works and
often breaks.
sub: remove old style override option

Didn't work too well.
Merge branch 'master' of https://github.com/mpv-player/mpv
vo_opengl: Make approx-gamma affect OSD/sub

Close #837