Don't override an entity set inside of updateEach

[krispya]

updateEach works ergonomically by generating a snapshot that gets passed into each callback and then the snapshots are written back to the store. This allows the store to be designed in ways that are optimized for storage, for example a structure of arrays, while still having an object-oriented interface for mutation.

// Position and Velocity traits are updated as expected from the mutation
world.query(Position, Velocity).updateEach(([position, velocity]) => {
    position.x += velocity.x * delta;
    position.y += velocity.y * delta;
});

However, there is an unintuitive interaction that occurs currently where you can use the entity's set method to set the trait values from inside the loop, and then the untouched snapshot gets written back to the store, overwriting any changes made via the entity set method.

// This will confusingly do nothing
world.query(Position, Velocity).updateEach(([position, velocity], entity) => {
    entity.set(Position, 
        (prev) => ({
            x: prev.x + velocity.x * delta,
            y: prev.y + velocity.y * delta,
        })
    );
});

The proper fix, I think, is to skip writing the snapshot back to the store if an entity set is called for that trait. This will require setting dirty flags, which is already supported with some refactoring.