perf: Add zIndexChanged$ + Improve pointer system perf

[eonarheim]

===:clipboard: PR Checklist :clipboard:===

• 📌 issue exists in github for these changes
• 🔬 existing tests still pass
• 🙈 code conforms to the style guide
• 📐 new tests written and passing / old tests updated with new scenario(s)
• 📄 changelog entry added (or not needed)

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This PR adds zIndexChanged$ to the ex.TransformComponent in order to know when the z index changes. This allows the ex.PointerSystem to maintain a sorted list which is much faster than using the ECS Query.