hwcomposer: Synchronization fixes

[aleasto]

No description provided.

[aleasto]

@dos1

This almost works. But sometimes it breaks when switching from full-ui mode to single-app mode

[aleasto]

Well, that is caused by contents->retireFenceFd, not the layers one.

[aleasto]

So, I believe this logic is correct.

However it relies on the assumption that the compositor will always reliably send a wl_buffer::release event, even when replacing a surface's buffer with itself again. Mutter curiously seems to respect this, but i think it is not the ideal behaviour; indeed kwin and weston do it differently.

So I believe I'm just going to keep this here to be revisited, and for now disable the per-layer synchronization.

[aleasto]

Alternatively, since the buffer cache is very rarely used as most frames post-boot have the GEOMETRY_CHANGED flag, we could just remove it and then wl_buffer::release should behave how this logic requires.