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 += () ...

CelesteNet/CelesteNet.Client/Components/CelesteNetMainComponent.cs

Line 815 in f80b8f0

scene.OnEndOfFrame += () => {

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:

CelesteNet/CelesteNet.Client/Components/CelesteNetMainComponent.cs

Line 578 in f80b8f0

level.OnEndOfFrame += () => ghost.Active = true;

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.

[RedFlames]

Oh I can add these types of comments too, eh.

Forgot to mention this line probably needs to be changed to something like:

if (player != null && player.Scene != Player?.Scene) {

just so we don't delete all Ghosts just because Player is null, probably happens somewhere during respawning.

[RedFlames]

Actually my suggestion would change/break Cleanup() behavior, which sets Player to null and removes all Ghosts -- with my suggestion, the Ghost removal wouldn't happen anymore...
Maybe we should seperate the Ghost removal from ResetState?