using the MPContext as ta parent was a bad idea and shouldn't be done in
any circumstances there because it only supposed to be for internal
usage. this had the undesired effect that the options group was freed
but still used since the MPContext is freed afterwards.
instead manually free options group.
all the get_property_* usages were removed because in some circumstances
they can lead to deadlocks. they were replaced by accessing the vo and
mp_vo_opts structs directly, like on other vos.
additionally the mpv helper was split into a mpv and libmpv helper, to
differentiate between private and public APIs and for future changes
like a macOS vulkan context for vo=gpu.
