Allow the window to keep updating during move/resize on Windows and macOS

On Windows, the SDL event loop would get blocked during move/resize,
which made the game stop updating. Since version 2.30, SDL will send its
SDL_WINDOWEVENT_EXPOSED event on an internal timer during window
move/resize so the application can keep redrawing.

Since no window size changed events can be received either, the size of
the window and the renderer is now updated at the start of each frame.

On macOS the SDL event loop was similarly blocked during resize.

The event watcher is only used on Windows and macOS because on X11 the
processing of all the queued up expose events can cause a delayed response to
resizing. Besides, the issues fixed by this watcher don't exist on X11 and
Wayland.

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