Add AlwaysPresent for AudioContainer and fix AlwaysPresent not being taken into consideration when computing isMaskedAway

[Drison64]

used for ppy/osu#29215

[smoogipoo]

Presence has nothing to do with being masked away or being alive. I'm fine with the property, if that's all that you need to get things working. Otherwise, we'll need to think of something else.

[Drison64]

I did not notice I pushed the ShouldBeAlive (was from when I was testing things out), i removed it and it still works.

I added alwaysPresent check to ComputeIsMaskedAway, because when it was masked away (in this case, off the screen) a check in CompositeDrawable line 917 does not pass. I am open to creating a different property just for this case if needed.

[smoogipoo]

Yeah, that check is supposed to not pass. It has caused problems in the past but I don't think presence is the way to fix this, or at least I'm not ready to do it yet.

There are two solutions I can mention without having looked at the osu!-side code:

1. Make the container not be masked away in general.
2. Override UpdateSubTreeMasking() and return false.

[peppy]

Also agree here.

If anything, rather than exposing this as a setting it should be implicitly applied to AudioContainer without the user having control.

[smoogipoo]

I'm not sure about that... Means if at some point the entire game is wrapped in an AudioContainer no masking calcs would ever take place. Would prefer it to be applied to a local container where the intent is explicit, for now.