command: list filters/VOs/AOs with option-info

Another special-case, but pretty simple after all.
vo_opengl: allow setting different filters for downscaling
demux_mkv: adjust subtitle preroll again (2)

Make the changes started in commit c827ae5f more eloborate, and provide
an option to control the amount of data read before the seek-target. To
achieve this, rewrite the loop that finds the lowest still acceptable
target cluster. It is now searched by time instead of file position. The
behavior (both with and without preroll option) may be different from
before this change, although it shouldn't be worse.

The change demux_mkv_read_cues() fixes a bug: when seeking after playing
normally, the code would erroneously assume that durations are set. This
doesn't happen if the first operation after loading was a seek instead
of playback.
sub: workaround braindead libass API

libass won't use embedded fonts, unless ass_set_fonts() (called by
mp_ass_configure_fonts()) is called. However, we call this function when
the ASS_Renderer is initialized, which is long before the .ass file is
actually loaded. (I'm not sure why it tries to keep 1 ASS_Renderer, but
it always did this.)

Fix by calling mp_ass_configure_fonts() after loading them. This also
means this function will be called multiple times - hopefully this is
harmless (it will reinit fontconfig every time, though).

While we're at it, also initialize the ASS_Renderer lazily.

Fixes #1244.
umpv: update comment for the new FIFO location
osc: add seekbarstyle option
osc: add validation for string user options
man: document osc seekbarstyle option
input.conf: Don't make ENTER exit the player

Apparently this is confusing.
lua: subprocess: add Windows implementation

Doesn't handle mp_cancel yet.