fix scaler options, add VOCTRL_SET_COMMAND_LINE

Well, this option-reparsing code sure is extremely weird, and it's just
a temporary solution anyway (should so something with m_property.c
instead...)

Also make sure there's always an osd object.
don't touch framebuffer until window is visible

This made trouble for wayland, and is actually completely unnecessary.
I don't quite understand why even glDrawBuffer(GL_BACK) raised an error,
though.
vo_xv, vo_x11: remove unused variable