Split hierarchy updates and transform propagation to two stages

[cyndis]

With hierarchy updates and transform propagation in the same stage,
GlobalTransform may fail to be updated when an entity is added
without using one of the .with_children style functions and Transform
is not changed on further frames. This can happen when creating an
entity with Transform and Parent directly.

The reason for this is that the transform propagation pass relies
on the Children component being set on the parent, but as the
parent update system is run in the same stage, the component
is not yet visible. On the following frame, the Changed
constraint filters out the entity, and as a result GlobalTransform
never gets updated.

Fix this by moving parent_update_system to a new UPDATE_HIERARCHY
stage just after the UPDATE stage (and likewise for startup stages).
This ensures that all systems in POST_STARTUP and further can rely
on the hierarchy being set up.

[cart]

Hmm while this does appear to resolve the issues mentioned, I think I'll hold off on merging for now. I don't find the "run systems in their own stage to enforce ordering" particularly satisfying. Its probably the best way to handle it with the tools we have today, but we have a number of fixes in the works:

• System sets and parallel executor v2 #1144 is a partial solve because it allows us to specify system dependencies, which means we can force hierarchy updates to run before transform updates (however its not a full solve because command buffers won't be applied between system executions)
• I'm currently working on support for alternative ECS storages and a hierarchy rework, which im pretty sure will resolve this nicely (because we should be able to cheaply insert components stored in Sparse Sets immediately from within a system)

That being said, lets leave this open for now because it solves real problems. If the approaches above don't pan out before the next release, we can merge this instead.

[cyndis]

Agreed, it's a hacky solution. Sounds good to me!

[MinerSebas]

Suggested change

	.add_stage_after(
	bevy_app::stage::UPDATE,
	stage::UPDATE_HIERARCHY,
	SystemStage::parallel(),
	)
	.add_stage_before(
	bevy_app::stage::POST_UPDATE,
	stage::UPDATE_HIERARCHY,
	SystemStage::parallel(),
	)

I think this is a better placement for the stage.

[cyndis]

This is now fixed in master through system ordering constraints -- closing.