output: use backend commits

[emersion]

Pass the whole new desired state to the backend, so that the
backend can leverage KMS atomic commits.

Depends on #313
See https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4548

[kennylevinsen]

Note: I think we need to do some work on the swapchain allocation to better reap the benefits of device-wide commits.

This currently picks a buffer type on each output in series, each blissfully unaware of the others' consequences to available memory bandwidth and shadow plane requirements as the resulting buffers are not committed. This seems like it would be quite a lot less likely to succeed than our existing setup, where each output commit at least observed the restrictions caused by the prior commits - and that's before considering the all-or-nothing commit we're now doing.

If we want to be able to bring up outputs in spite of modifier restrictions without needing users to set WLR_DRM_NO_MODIFIERS=1, we'd want a flow that looks like:

1. Set up swapchain with modifiers for all outputs
2. Try a backend-wide test and commit if successful
3. If failed, retry with modifierless swapchains for all outputs
4. Try a backend-wide test and commit if successful

This does seem like something one might want a helper for...

[emersion]

Yeah, that's a good point…

Backend-wide commits are still good to have even without the modifier fallback stuff: they provide atomic output configurations without some half-applied state in case of failures.

I'd prefer to keep the modifier fallback stuff as a follow up of the backend commits, but we will indeed either need to handle this in a helper or in wlr_backend_commit()…

[kennylevinsen]

Agreed.

I for fun tried to hack up a solution in clay (https://git.sr.ht/~kennylevinsen/clay/tree/dc82a8c/item/src/clay/output.c#L236), exposing a specialized version of wlr_output_configure_primary_swapchain (wlr_output_try_for_swapchain) that just grabs a buffer with the configuration you want, successful or not.

Seems to work quite well so far, so there's hope.

[emersion]

This PR now uses the new helper from wlroots!4567.