Draft: Preparations for wp_transaction and delayed surface commits

This is needed to implement the wp_transaction protocol and delayed surface commits in general. The latter can be useful for explicit sync.

• Figure out how to manage attached surface state. There are always two states - current and pending. With sub-surfaces and the wp_transactions protocol, we may sometimes copy(?) or move(?) the pending state to a cached state queue.

Idea 0:

• Define the SurfaceAttachedState as follows

  class SurfaceAttachedState
  {
  public:
      virtual ~SurfaceAttachedState() {}
  
      virtual SurfaceAttachedState *allocate() = 0;
      virtual void copyInto(SurfaceAttachedState *other) const = 0;
      virtual void moveInto(SurfaceAttachedState *other) = 0;
  };

  When we want to move the pending state to the cached state queue do

  SurfaceState *cached;
  pending->moveInto(cached);
  cachedQueue.enqueue(cached);
  
  // in SurfaceState::moveInto(SurfaceState *other)
  if (layerSurfaceV1) {
      if (!other->layerSurfaceV1) {
          other->layerSurfaceV1.reset(layerSurfaceV1->allocate());
      }
      layerSurfaceV1->moveInto(other->layerSurfaceV1);
  }

  When the pending state is applied, we just move the state from pending to current. In order to emit *Changed signals, we may need to make a copy of the current state before moving pending state into it.

Idea N+1: ??