Per-layer incremental updates

[emersion]

With https://github.com/emersion/libliftoff/issues/6, we try to re-use the previous plane allocation if (given the properties that changed) we won't be able to improve it.

However, if we think we can improve it, we throw the previous plane alloc away and start from scratch. Maybe we could keep as much as possible from the previous plane alloc, and only throw away the allocation for layers that changed.