Fix ghost life cycle edge cases

[Popax21]

This fixes a few edge cases related to ghost life cycles, which can cause ghosts to become invisible, frozen, or not rendered.

[RedFlames]

There seems to be a conflict between the loop for LastFrames in OnPlayerAdded being moved into scene.OnEndOfFrame += () ... https://github.com/0x0ade/CelesteNet/blob/f80b8f0186eb17e46594388f72d6d477515a1040/CelesteNet.Client/Components/CelesteNetMainComponent.cs#L815 and the fact that newly created Ghosts also do level.OnEndOfFrame += () => ghost.Active = true; with the result that a lot of spawned Ghosts are permanently disabled, presumably after being created inside this aforementioned loop. This is because you can't add to OnEndOfFrame from within itself, because Scene.AfterUpdate sets its OnEndOfFrame = null right after executing it, removing any newly added event handlers.

One fix that seems to work is to keep the new OnEndOfFrame but remove the one in CreateGhost, leaving new Ghosts disabled: https://github.com/0x0ade/CelesteNet/blob/f80b8f0186eb17e46594388f72d6d477515a1040/CelesteNet.Client/Components/CelesteNetMainComponent.cs#L578 and then doing this within CelesteNetMainComponent.Update:

            foreach (Ghost g in Ghosts.Values)
                if (g != null)
                    g.Active = true;

I don't know if this would need a flag on the Ghosts to only activate them once or if doing it unconditionally is fine.