Crash when creating actors from OnStart

[Gopmyc]

Description

Creating actors from a script's OnStart (same for OnAwake and OnEnable) can crash the engine when entering play mode.

Scene::Play loops over m_actors with iterators while calling these callbacks. If a script calls Scene:CreateActor there, m_actors can be reallocated and the loop keeps reading the old buffer, which ends in an access violation in Actor::IsActive.

It only crashes when the actor holding the script isn't the last one in the scene, because the loop stops right after the last actor. Scene::Update, FixedUpdate and LateUpdate don't have this problem since they iterate over a copy of m_actors. Doing the same in Scene::Play should fix it.

To Reproduce

Steps to reproduce the behavior:

1. Create a new scene (File > New Scene)
2. Create a script with the following content:

local Spawner = {}

function Spawner:OnStart()
    for i = 1, 10 do
        Scenes.GetCurrentScene():CreateActor("Spawned " .. i, "")
    end
end

return Spawner

3. Add the script to Main Camera, which is the first actor of the scene
4. Press Play
5. See the crash (access violation in Actor::IsActive)

Expected behavior

The actors are created and play mode starts normally, the same way it works when CreateActor is called from OnUpdate.

Screenshots

N/A

[Gopmyc]

The copy should be taken once, before the loops. CreateActor already calls OnAwake, OnEnable and OnStart on actors created during play, and these callbacks aren't guarded against being called twice, so a copy per loop would call OnEnable and OnStart twice on actors created from OnAwake

[Gopmyc]

@adriengivry I think this issue could get the "Good First Issue" tag. The fix is small and stays in Scene::Play, and the issue already explains the cause, so it's a good way for someone to take their first steps in the project
And since it's a fix for a current bug, it would probably be cool to include it in 2.0 as well