Race condition on `Registry.setSystemDeltaSeconds`

[Bruno Franca dos Reis]

Hi!

It seems that there's a race condition in Registry.setSystemDeltaSeconds, or I might be misunderstanding something.

This is the minimal example that reproduces the issue:

func TestRaceCondition(t *testing.T) {
	reg := ecsgo.New()
	ecsgo.AddSystem(reg, ecsgo.OnTick, func(r *ecsgo.Registry) {})
	ecsgo.AddSystem(reg, ecsgo.OnTick, func(r *ecsgo.Registry) {})
	reg.Tick(1.0)
}

Running the snippet above with the race detector reveals the race. The goroutines launched for each of the systems end up calling setSystemDeltaSeconds(...) which write to r.sysDeltaSeconds, but there's no synchronization.

Any suggestions?
Thanks!