Synchronize surfaces to a single output

[emersion]

Right now, if a surface is visible on two outputs, it'll be synchronized to both -- meaning that it'll be rendered twice too often. If both outputs have a 60Hz refresh rate, we'll send a frame event at 120Hz. This also causes issues with presentation-time, confusing clients with invalid refresh data.

Instead we should synchronize a surface to a single output (as suggested in the presentation-time protocol, as done by Weston). We can still render the surface on both.

I'm not sure how much of this should be in wlroots yet.

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wlroots has migrated to gitlab.freedesktop.org. This issue has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/1331