remove the libmpv observer for the macOS specific options and use a
config cache + change callback for runtime changes. this is also a
preparation for new backends and generalises even more, since libmpv
functions can't and shouldn't be used in usual vo backends. for feature
parity the config cache is used.
move all backend independent code parts in their own folder and files,
to simplify adding new backends. the goal is to only extend one class
and add the backend dependent parts there. usually only the (un)init,
config and related parts need to be implemented per backend. furthermore
all needed windowing and related events are propagated and can be
overwritten. the other backend dependent part is usually the surface for
rendering, for example the opengl oder metal layer.
in the best case a new backend can be added with only a few hundred
lines.
der richter