cocoa/libmpv: allow to embed mpv GL view in another window

This is just temporary code but is a good base for future work (and baby
steps are required for these changes). The 'final destination' is embedding
the video view into any NSView but that requires some more work (the mechanism
will be the same: pass the view's pointer casted to int64_t through -wid).

For instance we will need to remove as much usage of the window instance
as possible, and use nil guards where not possible. For this reason I will
remove stuff like the mission control fullscreen feature (it's a cute feature
but annoying to support and quite limited, go make your GUIs), and a way to
lookup the current screen directly from the NSView absolute coordinates
(this is needed for ICC detection mostly, and reporting back the screen to
mpv's core).

Moreover the current view.m will need to be separated into 2 views: the actual
video view that will be embedded, and a parent view that will not be embedded
and will be responsibile for tracking events.