This avoids bouncing the content between the map binding and DIB.

Its superclass, NSOpenGLContext, only holds a weak reference.  The view was
sometimes being deallocated before the context was disposed of, resulting in
crashes.

If the context is being made current with its existing view, just update it
before making it current.

We clear it if the context or the view is NULL.  If the context is non-NULL,
we want to disassociate the views of both the current and passed-in contexts,
if they differ.