Don't apply tonemapping for unlit material

[OmarShehata]

This PR skips the LINEARtoSRGB if the material is unlit. This seemed like a simpler approach than changing the LINEARtoSRGB function to be a no-op for unlit materials. @lilleyse let me know if you agree with this approach.

This also fixes the 2nd failing unit test (from #8127) by just updating the color.

You can test that this correctly renders an unlit material with:

var viewer = new Cesium.Viewer('cesiumContainer', {
    infoBox : false,
    selectionIndicator : false,
    shadows : true,
    shouldAnimate : true
});

//viewer.scene.highDynamicRange = true;

function createModel(url, height) {
    viewer.entities.removeAll();

    var position = Cesium.Cartesian3.fromDegrees(-123.0744619, 44.0503706, height);
    var heading = Cesium.Math.toRadians(135);
    var pitch = 0;
    var roll = 0;
    var hpr = new Cesium.HeadingPitchRoll(heading, pitch, roll);
    var orientation = Cesium.Transforms.headingPitchRollQuaternion(position, hpr);

    var entity = viewer.entities.add({
        name : url,
        position : position,
        orientation : orientation,
        model : {
            uri : url,
            minimumPixelSize : 128,
            maximumScale : 20000
        }
    });
    viewer.trackedEntity = entity;
}

createModel('../../../Specs/Data/Models/PBR/BoxUnlit/BoxUnlit.gltf', 5000.0);

I used the browser's color picker to verify it is indeed the green base color.

[cesium-concierge]

Thanks for the pull request @OmarShehata!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[mramato]

@lilleyse please review/merge ASAP so we can get the release out.

[lilleyse]

The linear to SRGB conversion still needs to be applied. Only the tonemapping portion should be skipped if unlit.

This branch (with hdr off by default)

1.60 with HDR off

[lilleyse]

I'm working on a fix...

[lilleyse]

Hm just realized that this tileset is not using the unlit extension. I'll have to open a separate issue for that.

[lilleyse]

My changes are up. I'll open a separate PR for AGI HQ updates.