Fix scene.pickPosition when depthTestAgainstTerrain is false

[ggetz]

Description

Fixes inaccurate results for scene.pickPosition when depthTestAgainstTerrain is false.

Although we were copying the initial globe depth before clearing the globe for subsequent passes, the globe depth was not getting copied into the pickDepth buffer.

This updates pickDepth to resolve depth from both the original and updated globe depth values.

Issue number and link

Fixes #4368

Testing plan

1. Here's a testing Sandcastle that clearly illustrated the initiation problem. It outputs the results of pickPosition vs globe.pick. The values are wildly different before the fix, and should be similar after.
2. Copy/paste the picking code into various examples to confirm:

• Scene with globe and model
• Scene with globe and tileset
• Scene with translucent globe
• 2D scene
• Scene with classification

// Mouse over the globe to see the cartographic height
const handler = new Cesium.ScreenSpaceEventHandler(viewer.scene.canvas);
handler.setInputAction(function (movement) {
  const scene = viewer.scene;
  if (scene.mode !== Cesium.SceneMode.MORPHING) {
      let cartesian = scene.pickPosition(
        movement.endPosition
      );
    
    

      let heightString = "";
      if (Cesium.defined(cartesian)) {
        const cartographic = Cesium.Cartographic.fromCartesian(
          cartesian
        );
        const longitudeString = Cesium.Math.toDegrees(
          cartographic.longitude
        ).toFixed(2);
        const latitudeString = Cesium.Math.toDegrees(
          cartographic.latitude
        ).toFixed(2);
       heightString += cartographic.height.toFixed(2);
      }
    
    heightString += " | ";
    
    
    cartesian = scene.globe.pick(scene.camera.getPickRay(movement.endPosition), scene);
    
    if (Cesium.defined(cartesian)) {
        const cartographic = Cesium.Cartographic.fromCartesian(
          cartesian
        );
        const longitudeString = Cesium.Math.toDegrees(
          cartographic.longitude
        ).toFixed(2);
        const latitudeString = Cesium.Math.toDegrees(
          cartographic.latitude
        ).toFixed(2);
       heightString += cartographic.height.toFixed(2);
      }
    
    console.log(heightString);
    }
}, Cesium.ScreenSpaceEventType.MOUSE_MOVE);

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have update the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[ggetz]

@lilleyse Can you take a look please?

[github-actions]

Thank you for the pull request, @ggetz!

✅ We can confirm we have a CLA on file for you.

[lilleyse]

I'm always a little concerned about changes like this, but I couldn't think of any cases where it would break. Hopefully two weeks in main is enough time to catch any issues.

The code looks good though. I like that it's a simple shader change.