Billboard depth testing performance improvements

[hpinkos]

Fixes #6699

Moves the billboard depth test logic to the vertex shader when texture reads from the vertex shader are supported. Otherwise it falls back to doing the depth test in the fragment shader.

[cesium-concierge]

Thanks for the pull request @hpinkos!

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• ❔ CHANGES.md was not updated.
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[hpinkos]

@bagnell can you review?

[hpinkos]

Here's the sample code from my last PR:

var viewer = new Cesium.Viewer('cesiumContainer', {
    terrainProvider: Cesium.createWorldTerrain()
});
viewer.scene.globe.depthTestAgainstTerrain = true;

var e = viewer.entities.add({
    position : Cesium.Cartesian3.fromDegrees(-122.1958, 46.1915),
    billboard : {
        image : '../images/Cesium_Logo_overlay.png',
        heightReference : Cesium.HeightReference.CLAMP_TO_GROUND,
        verticalOrigin: Cesium.VerticalOrigin.BOTTOM
    }
});

viewer.trackedEntity = e;

var viewer = new Cesium.Viewer('cesiumContainer', {
    terrainProvider: Cesium.createWorldTerrain()
});
viewer.scene.globe.depthTestAgainstTerrain = true;

var e = viewer.entities.add({
    position : Cesium.Cartesian3.fromDegrees(-122.1958, 46.1915),
     label : {
        text : 'Clamped to ground',
        heightReference : Cesium.HeightReference.CLAMP_TO_GROUND,
        horizontalOrigin : Cesium.HorizontalOrigin.CENTER,
        verticalOrigin : Cesium.VerticalOrigin.BOTTOM,
        fillColor : Cesium.Color.BLACK,
        showBackground : true,
        backgroundColor : new Cesium.Color(1, 1, 1, 0.7),
        backgroundPadding : new Cesium.Cartesian2(8, 4)
    }
});

viewer.trackedEntity = e;

[bagnell]

When the origin is the top right, it seems like it is only depth testing that point. You can see that the top left is not behind any terrain.

[hpinkos]

Thanks @bagnell, I think this is ready now

[bagnell]

@hpinkos Just had some formatting changes in ce49ae9. Everything else looks good. Thanks!