Simplify how infinite input regions are handled
Some input related code in kwin is mislead by the fact that when the input region is infinite, SurfaceInterface::input() is going to return an empty QRegion object. We cannot really do that because the client could have just set a valid empty wl_region object to ignore all input events. This change makes SurfaceInterface assign an actually infinite region when a NULL input region has been passed to set_input_region().
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