Name the field "sub_id" as it's not specific to DVD subs.
Remove some other unused extern declarations together with dvdsub_id
from demux_mkv.c and demux_lavf.c.
Add a 'struct vo *vo' argument to the x11_common.c functions that
access the variable so it's available as vo->opts->vo_ontop. To keep
VOs using the old API working create a global vo variable that is set
to the currently used old vo. "vo_ontop" will be #defined to
"global_vo->opts->vo_ontop", and x11_common.h will add defines like
the following when it is included by old VOs:
#define vo_x11_ontop() vo_x11_ontop(global_vo)
so that they will call the function according to the new declaration.
First part of option restructuring. The aim is to move option values
from a huge number of separate globals to a single non-global struct.
This part adds some support for parsing option values into such struct
instances, and moves one example option (fixed-vo) to the struct.
