options: fix positional suboption asrguments

E.g. "-vf scale=848:480" set the w argument twice, instead of setting w
and then h.

This was caused by accidental shadowing of a local variable.

Regression since probably 4cd143e.
Enable -Wshadow

This one really did bite me hard (see previous commit), so enable it by
default.

Fix some cases of shadowing throughout the codebase. None of these
change behavior, and all of these were correct code, and just tripped up
the warning.
demux_mkv: use a more universal zero initializer

Unfortunately, we can't avoid this warning 100%, because ebml_info is
written by a Perl script. I think the script writes the struct fields in
random order (thanks Perl), so there's no way to know whether the first
struct field is a scalar or a struct.

At least {0} is always valid here, even if it shows a warning. (The
compilers are wrong, see e.g. [1].)

[1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119