osu-lazer

Fix subscription leak from composer reloads (#36828 )

The reproduction scenario for the subscription leak is as follows:

1. Switch to a scrolling ruleset (anything but osu! from the standard
ones).
2. Select a beatmap to edit.
3. Load the composer.
4. Go to timing tab.
5. Change a timing point.
6. Go back to the composer.

At this point, `EditorChangeHandler.OnStateChange` will have multiple of
the same delegate in the invocation list.

That in turn is caused by the fact that changing a timing point *does*
incur a full reload of the composer via the following flow:

https://github.com/ppy/osu/blob/15b6e28ebe888b1a87574891be1a0db3b04093b7/osu.Game/Rulesets/Edit/ScrollingHitObjectComposer.cs#L145
https://github.com/ppy/osu/blob/64a29313a852d50095ae4b7ea8f22fde23aa634f/osu.Game/Screens/Edit/Editor.cs#L1137-L1145

This flow is my "fault"; see https://github.com/ppy/osu/pull/28444. The
reason why a full composer reload is used is not clear to my
recollection at this time, but it is likely because it's just the least
likely to fail. A smarter solution that wouldn't require a full reload
would also entail checking that there exists a safe insertion point that
allows replacing timing points in a way that will reflect everywhere it
must. Including all of the `IScrollingAlgorithm` machinery and such.

In general it is not uncommon in the codebase to not bother to clean up
some event callbacks if it is implicitly or explicitly guaranteed that
both objects bound by the callback will always get disposed in tandem at
the same time. This *was* true with this particular flow to a point,
which was until that full composer reload was implemented.

<details>
<summary>To address the elephant in the room</summary>

Someone will inevitably notice https://github.com/ppy/osu/pull/36824
which was a clanked pull request pointing out this leak. And then
someone will inevitably call this "AI discrimination"! *Gasp!*

So first of all, let me stop you right there. Yes, as far as I am
_personally_ concerned, it is "AI discrimination". I invoke the full
force of the Butlerian Jihad.

The clank army's goal is to eradicate my job and make me work in an
Amazon warehouse instead. Or, if not that, at least my job is to be rid
of all remnants of fun I still get from it and for me to be reduced to
that one guy from the meme "i guess we're doin circles now". You know
the one.

I resent this. You attack me directly. I do not perceive the need to
meet you halfway or be civil.

That said, I have too much respect for the users of this software to
leave reports of potentially real issues unchecked. So I did check, and
it was real. And you know what? Good job to the clanker. It did what it
was designed to do: it parsed a code file, recognised a hole in a
pattern it was designed to recognise, and invoked forms of language
given to it to communicate this to the meatbag that opened that PR.

And here's the thing: my primary issue is with that meatbag that opened
that PR. That meatbag served no functional purpose in any of this. The
meatbag took a hose that spews 90% water and 10% raw sewage at random
intervals and pointed it at my house directly, claiming that they just
want to clean it. At no point did the meatbag appear to have the common
decency to pull out a container, pour some magic liquid out, check if
there's sewage in it, and filter it out if there is any. But no, that
would take *effort* and *thought*, would it not? The *effort* and
*thought* that is required of *me* to *review* the clanker's work?

The PR had no reproduction scenario, and had testing checkboxes that
were presumably meant for *me* to check off. Why is it *my* job to
figure all of this out rather than the submitter meatbag's?

I do *not* have obligations towards spew-hose-pointing meatbags. Point
that hose at your own backyard at your peril.

If you *actually manage* to get the clanker to filter out *all* of the
spew without fail itself, my only win condition is gone. But it is not
yet that time. So at least have the decency to check for the spew
yourself, rather than telling the clanker to put checkboxes in the PR
descriptions telling *me* to check for it.
</details>