wgl: Eliminate the shared layer; implement WGL API on top of the ICD callbacks.

wgl: Eliminate the shared layer; implement WGL API on top of the ICD callbacks.

While the WGL API has been stale for decades now, the ICD interface has
been updated with new Windows versions, so it is much easier to define
everything in terms of the ICD interfaces, which is pretty much what
Microsoft's opengl32.dll does anyway.