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.
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.