Avoid double-rendering on resize

On Windows and macOS, resizing the window with the mouse blocks the
event loop until the click is released. To keep rendering during
resizing, an event watcher is registered to handle resize events.

Avoid handling resize events twice by ensuring
`SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED` is processed only once.

PR #6758 <https://github.com/Genymobile/scrcpy/pull/6758>