Mechanism to wait for page-flip before reusing plane on different CRTC

Not sure if this is the same issue described in #34, but it sounded a bit different.

What I'm trying to fix in !85 is a bit more complicated than I initially thought. I think my EBUSYs are happening when a commit on output A frees up a plane and then a commit on output B tries to use the same plane before output A has done a page-flip. As far as I can see there's nothing in libliftoff/wlroots to prevent this

I propose: (better naming suggestions welcome)

• Add to struct liftoff_plane a new free_after_pageflip field pointing to the output the plane used to be attached to. This gets set in liftoff_output_apply() when clearing layer-plane mappings.
• Add a new void liftoff_output_pageflip(struct liftoff_output *output) function which clears free_after_pageflip for any planes matching the output.
• Make liftoff_output_apply() not use any planes which have free_after_pageflip set to an output other than the current one

How does that sound? If we wanted to make the change non-breaking then there could be a new liftoff_output_apply_pending() with the new behaviour while the existing liftoff_output_apply() doesn't set free_after_pageflip.