Skip setting properties that haven't changed

[emersion]

~~Atomic test-only commits set property values for the next commit when they succeed.~~ We don't need to keep setting the same property values over and over again.

[emersion]

Atomic test-only commits set property values for the next commit when they succeed.

Nope, that's false. Was confused because wlroots re-used the atomic requests between commits.

Only regular commits set property values for subsequent commits.

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Another thing to keep in mind is that a VT switch messing up our previous state may happen at any time. Also there's no guarantee the compositor has performed a regular atomic commit after liftoff_output_apply.