layer-shell-v1: introduce wlr_layer_surface_v1_state.committed
This allows compositors to figure out which state fields have changed since the last commit, and avoid needlessly re-computing their configuration on each commit.
Not usable as-is, because client_pending.committed
is reset before commit, and current.committed
is not well-defined. Ref https://github.com/swaywm/wlroots/issues/2098