GDK-Win32: Fixup, and cleanup WGL HDC handling

GDK-Win32: Fixup, and cleanup WGL HDC handling

It turns out that the problem of the WGL window not drawing was due to
the fact that I messed up where I placed SwapBuffers() during the
conversion... doh:|

At the same time, stop storing the HDC in the GdkWin32GLContextWGL, but
instead always create it along the surface we created, so that it is ready
for use for operating with WGL when we are not dealing with "surfaceless"
contexts.  If we are dealing with "surfaceless" contexts, just use the
HDC of the dummy window that we created when we created the
Gdk(Win32)Display.

WGL contexts should now be in working order at this point.