Keep track of the default values directly, instead of creating a new
instance of the option struct just to get the defaults.
Also get rid of the special handling of m_obj_desc.init_options.
Instead, handle it purely by the option parser. Originally, I wanted to
handle --vo=opengl-hq and --vo=direct3d_shaders with this (by making
them aliases to the real VOs with a different preset), but since --vo
=opengl-hq=help prints the wrong values (as consequence of the
simplification), I'm not doing that, and instead use something
different.