LabelGraphics of an entity goes above the entity billboardGraphics when showBackground set to true

The issue is that labelGraphics of an entity goes above the entity billboardGraphics when showBackground set to true.

The required view to reproducing the bug is 2D.

First example:
[LabelGraphics of an entity goes above the entity billboardGraphics when showBackground set to true (unexpected behavior).](https://sandcastle.cesium.com/#c=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)

Second example:
[ labelGraphics of an entity goes under the entity billboardGraphics when showBackground set to false (expected behavior).](https://sandcastle.cesium.com/#c=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)

Cesium view: 2D

Browser: Chrome

Operating System: Windows

Open both linked Sandcastle reproductions in Cesium's 2D view and compare the entity's LabelGraphics and billboardGraphics with showBackground true and false. Trace the relevant label and billboard rendering entry points, then verify that the label remains below the billboard when showBackground is true while preserving the expected false behavior.

Written by the indexing model from the issue text.