Improve incrementality of transform propagation updates for siblings

[cart]

As of #4608 (for legitimate correctness reasons), adding or removing any child now results in recomputing the transform hierarchy for every sibling. This is extremely suboptimal for large scenes. For example, despawning a single "house" entity in a scene with thousands of houses should not recompute the transform for every house (and the things inside of it). Given that transform propagation is often a bottleneck, fixing this should be a priority.

[alice-i-cecile]

Added to the 0.8 milestone; I agree that we should aim to get this fixed ASAP.

[yilinwei]

Apologies, I haven't quite followed the discussion on Github or Discord; but what was the proposed fix going to be since adding a marker component was seen as untenable?

Was it going to be the proposed rfc?

[alice-i-cecile]

what was the proposed fix going to be since adding a marker component was seen as #3340 (comment)?

If we can get better performance characteristics with a marker component, I'm alright with that as a stop-gap.

If we can get either the scheduling changes or a hierarchy redesign in in time for the next release, I'd prefer a more elegant and robust approach instead.

[alice-i-cecile]

@cart @james7132, this is in the milestone. Is this still relevant following #4197?

[james7132]

Definitely still an issue. Any change to Children leads to propagating through the entire subtree.

[cart]

This won't be solved / reviewed in time for 0.8. But should be prioritized / not forgotten. For now I'm adding a P-High so we don't lose it. We should also add this to the 0.9 milestone when that is created.