dix: Skip more code in SetRootClip for ROOT_CLIP_INPUT_ONLY

Despite e957a2e5 ("dix: Add hybrid full-size/empty-clip mode to SetRootClip"), I was still seeing all X11 client windows flashing when the root window size changes with rootless Xwayland (e.g. due to hotplugging a monitor).

Skipping this code for ROOT_CLIP_INPUT_ONLY fixes the issue for me.

/cc @daniels

Maybe this is just a gross hack?

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