Polyline z index doesn't work in 2D

This [came up on the forum](https://groups.google.com/d/msg/cesium-dev/cAreNI5IO-k/_VQEuC0BCAAJ). You can see it in the [z-indexing Sandcastle](https://sandcastle.cesium.com/#c=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) where I just changed the polyline's z-index to -2 instead of 2 so it's behind everything. In 3D it's correctly behind the other ground primitives. In 2D it goes back on top.

Start by running the linked z-indexing Sandcastle example and compare the polyline ordering in 2D and 3D, using the forum report for context. Done means a polyline with z-index -2 remains behind the other ground primitives in 2D as it does in 3D.

Written by the indexing model from the issue text.