Show spectating users during gameplay

[bdach]

• Depends on Implement spectator list display #31526

	argon	triangles	classic
osu!
taiko
catch
mania

Please forgive the playfield on the above looking a bit broken, it's what happens when the skin doesn't fully correctly reload during pause. That was the quickest way to capture these screenshots. (By the way this sort of thing is what needs to happen now that every ruleset can have elements in arbitrary places...)

[peppy]

I find it super odd that these bindings are done here. I get that this is to allow the base class to be tested, but it doesn't sit well. Two proposals:

• Rename this class to OnlineSpectatorList (dunno)
• Change how the testing is done to not require that and move the ISerialisableDrawable to the base implementation

Leaning towards the second.

[smoogipoo]

Because this is a threaded component, these should not be bound on the BDL thread. Especially and in particular the BindableList<>.

There is a potential race condition where:

• This thread is in the middle of copying elements from the remote list on the BDL thread.
• The remote list changes on the Update thread.

I believe these days we use LoadComplete() to bind bindables in general.

Here's an example:

[Test]
public void RunTest()
{
    var list1 = new BindableList<int>([0]);

    new Thread(() =>
    {
        int nextAdd = 1;
        int nextRemove = 0;

        while (true)
        {
            if (RNG.Next() % 3 == 0)
                list1.Remove(nextRemove++);
            else
                list1.Add(nextAdd++);
        }
    })
    {
        IsBackground = true
    }.Start();

    new Thread(() =>
    {
        while (true)
        {
            var list2 = new BindableList<int>();
            list2.BindTo(list1);
        }
    })
    {
        IsBackground = true
    }.Start();

    while (true)
    {
        Thread.Sleep(2000);
    }
}