Fusion.Children should parent things in batching order about fusion

[boatbomber]

Note that this change doesn't "solve" batching drawcalls, just some cases. If your images are nested inside wrapper frames, this change has no impact, for example. Your mileage may vary, but if your tree is pretty flat this can have some real benefit.

[dphfox]

Since discovering that ZIndex can control batching without the library having to add hardcoded behaviour for this, is it necessary for us to account for this detail ourselves?

[boatbomber]

I would argue that yes, it does still help. Managing ZIndexes to layer things for batching is a headache to do in every component (you'd need to create your Children array in parenting order, which is annoying and unwieldy), and there are cases where you can't do that anyway.

[boatbomber]

Since making this issue, I've pushed this implementation to production and have had no problems with it. My drawcalls decreased by 20%, although that does include some additional work I did to restructure some trees to benefit from this.

[boatbomber]

I opened the PR to show what that implementation looks like, but if we decide not to do this then I don't mind closing it. I do believe that this simple change provides value though.

[dphfox]

I mentioned this in the PR, but I'm going to defer on this for a while until we know whether Roblox themselves plan to address this soon.

[dphfox]

Unblocking this; see #298.