allow to explicitly define the z-order

[JHNP727]

Objective

This allows to specify an explicit z value for ordering instead of relying on the z translation value which might already be used for actual positioning.

Solution

We add an extra optional field that can be set. The default behavior of using the z translation does not change.

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Changelog

Added

• Allows to specify an explicit sprite ordering value when using the 2D renderer

[github-actions]

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[alice-i-cecile]

Related to #1275.

[alice-i-cecile]

Very much in favor of moving away from using the Transform's z value for ordering 2D objects, one way or another.

It feels like this would be better suited as a distinct component (so then it can be reused across sprites and UI, and avoid increasing the size of Sprite) rather than as a field on Sprite.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[CMorrison82z]

Status ? I don't think using Transform for this is a good idea as it could have unintended side effects. Rather, I think it should affect to Bevy Render order instead

Edit : I currently create another entity for the sprite and set the parent to the desired entity and this would avoid most side-effects