`MorphWeights` and `MeshMorphWeights` have been restructured

Mesh morph target weights have been restructured to improve flexibility and performance. Users who manually create MeshMorphWeights or MorphWeights components may need to make changes.

In Bevy 0.18, entities with a Mesh3d component could have a MeshMorphWeights component containing morph weight values. In addition, if a parent of the mesh entity had a MorphWeights component then its values would be automatically copied to the MeshMorphWeights component - this allowed multiple meshes to share a single set of weight values.

In Bevy 0.19, MeshMorphWeights has been changed. It can now be either a set of weight values as before, or a reference to an entity containing a MorphWeights component. Referencing replaces the previous automatic copying.

// 0.18
struct MeshMorphWeights { weights: Vec<f32> }

// 0.19
enum MeshMorphWeights {
    Value { weights: Vec<f32> },
    Reference(Entity),
}

If you were using MeshMorphWeights on its own, then you just need to use MeshMorphWeights::Value.

If you were using MorphWeights and MeshMorphWeights and relying on the automatic copying, then you need to use MeshMorphWeights::Reference and point it to the entity with MorphWeights.

// 0.18
parent_entity.insert(MorphWeights::new(...));
mesh_entity.insert((mesh, MeshMorphWeights::new(...)));

// 0.19
parent_entity.insert(MorphWeights::new(...));
mesh_entity.insert((mesh, MeshMorphWeights::Reference(parent_entity)));

These changes improve performance due to less copying. They also add flexibility - a MeshMorphWeights component can reference a MorphWeights component on any entity, not just its parent.

As a result of these changes, MorphPlugin was no longer needed and has been removed.