viewbox.rs: Use gboolean, not bool for field in a repr(C) struct
viewbox.rs: Use gboolean, not bool for field in a repr(C) struct

See https://github.com/rust-lang/rfcs/issues/992 - bool in Rust does not
repr(C) as any C type in particular.  I was naively assuming that it
would repr(C) as int, which is equivalent to gboolean.

This makes tests/fixtures/dimensions/bug612951.svg pass again.  We were
calling rsvg_node_svg_get_view_box() and getting a garbage value inside
vbox.active - the low byte actually set as expected, and the high bits
set to garbage.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg