Whether this property can only be modified when a Particle is emitted.
Set to true to allow only Phaser.GameObjects.Particles.EmitterOp#onEmit callbacks to be set and
affect this property.
Set to false to allow both Phaser.GameObjects.Particles.EmitterOp#onEmit and
Phaser.GameObjects.Particles.EmitterOp#onUpdate callbacks to be set and affect this property.