Avoid using user32.
-- v2: winemac: Directly use win32u for user functions in window.c. winemac: Directly use win32u for user functions in opengl.c. winemac: Directly use win32u for user functions in mouse.c. winemac: Directly use win32u in more places. winemac: Directly use win32u for user functions in clipboard.c. winemac: Directly use win32u for user functions in display.c.