cocoa: redo synchronization

Before this change, Cocoa state was accessed from both the VO and the
Cocoa main thread. This was probably not a good idea. There was some
locking as well as implicit synchronization using the dispatch
mechanism, but it wasn't watertight.

Change this completely. Now Cocoa things are always accessed from the
main thread only. The old mutex falls away, as well as the
vo_cocoa_set_current_context() function, which implicitly used the lock
to coordinate VO accesses. With the new code, the VO thread generally
has to wait for the main thread, while the main thread never waits for
the VO and rarely accesses it. Fortunately, this is rather straight
forward, and most of this is achieved by making vo_cocoa_control() run
on the main thread. The logic of the code does generally not change.

Some aspects are trickier. Apparently we can't access the
NSOpenGLContext from the VO thread, because this object is not thread-
safe. We use some CGLContextObj functions instead, such as for making
the context current and swapping the buffers.

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