WinGraphicsMode no longer creates a temporary context in order to create
the list of available modes. Instead, it requires to be passed an
existing context in its constructor.
WinGLContext now creates one temporary context in its static constructor
and hands that to WinGraphicsMode.
WinFactory no longer supports the CreateGraphicsMode API. This API will
be removed in the future, because the link because contexts and modes
cannot be separated in the general case.
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