output: add wlr_output_state.allow_artifacts

When starting up, the compositor might call wlr_output_set_mode()
with a mode which is already the current one. wlroots will detect
this and make the wlr_output_set_mode() call a no-op. During the
next wlr_output_commit() call, wlroots will perform an atomic
commit without the ALLOW_MODESET flag.

This is an issue, because some drivers need ALLOW_MODESET even if
the mode is the same. For instance, if the FB stride or modifier
changed, some drivers require a modeset.

Add a new flag "allow_artifacts" which is set when the compositor
calls mode-setting functions. Use this flag to figure out whether
we want to perform atomic commits with ALLOW_MODESET.

(The name "allow_artifacts" is picked because ALLOW_MODESET is a
misnomer, see [1].)

[1]: https://patchwork.freedesktop.org/patch/505107/

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3499

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