Resize sdl2 windows properly

This pr takes care of resizing the sdl2 window properly when the Window.size is changed. Previously only the displaymode was changed (not the physical window size), so the drawing area didn't match the window size.

I still don't understand exactly what the displaymode is important for, since update_viewport seems to take care of making sure the gl drawing region always matches the window. Maybe the new resize_display_mode should also be called with resize_window?

Would resolve https://github.com/kivy/kivy/issues/2519