I think this dates from a time where every window had its own window surface, and unless I'm missing something this isn't needed anymore. My intention is to refactor the client surfaces, so that they can be manipulated directly from win32u, and unify GL/VK client surfaces. This would make the code simpler, in preparation for that.
-- v2: winemac: Get rid of now unnecessary child cocoa views. winemac: Use the new client surface views for GL rendering. winemac: Create new client views with each VK/GL surface.