Fix ClientRectangle setter on Windows (WinGL) & Linux (X11)

This pull request fixes the ClientRectangle to not ignore the passed rectangle's position. This is needed for fixing https://github.com/mono/MonoGame/issues/1414.

I'm submitting this pull request here as it seems to be the most up-to-date Git clone of OpenTK used for MonoGame. Not sure if it's the right place or if we should wait on mono/OpenTK to be updated instead.