Fix entity show issues, issue #7048

[cleroux]

Addresses issue #7048
Fixes a regression caused by #6835 which resolved issues #2909, #4714, #6830, and #5371.

Reverted #6835 locally and confirmed each issue.
Applied these changes and confirmed correct behavior for each of the listed issues.
Ran all tests before and after changes. No new test failures reported.
Additionally ran many of the Sandcastle examples and observed no abnormal behavior.

[cesium-concierge]

Thank you so much for the pull request @cleroux! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

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[cleroux]

My individual CLA has been sent.

[pjcozzi]

Thanks again @cleroux, we received your CLA.

[hpinkos]

Thanks for the PR @cleroux! I'm glad you were able to track down the source of the problem. As you mentioned in your comment in #7048, that code was there for a reason, but I can't remember exactly what off the top of my head. I'll take a closer look so we can merge this fix in for the next release without introducing a new regression.

[hpinkos]

@cleroux upon further review, I think your change is the right thing to do here. I believe this used to work differently, but pretty much what's happening now is that block you removed was setting the attribute value of the new geometry instance to the attributes of the old geometry instance which is removed when the new one is ready. This is what was causing the flickering. However, at the end of the block we blow away all of the attribute values anyway so there was no point in updating them. https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/DataSources/StaticGeometryColorBatch.js#L190

Thanks so much for tracking this down!

[OmarShehata]

This also fixes the crash described here:

https://groups.google.com/d/msg/cesium-dev/1LHJQatvBZk/jbobj_-SCQAJ

which is reproduced by:

var viewer = new Cesium.Viewer('cesiumContainer');

viewer.screenSpaceEventHandler.setInputAction(function onLeftClick(movement) {
     viewer.entities.getById("1").polygon.material.color = Cesium.Color.BLUE;
}, Cesium.ScreenSpaceEventType.LEFT_CLICK);

var redPolygon = viewer.entities.add({
    name : 'Red polygon on surface',
    id: '1',
    polygon : {
        hierarchy : Cesium.Cartesian3.fromDegreesArray([-115.0, 37.0,
                                                        -115.0, 32.0,
                                                        -107.0, 33.0,
                                                        -102.0, 31.0,
                                                        -102.0, 35.0]),
        material : Cesium.Color.RED
    }
});

for(var i=0;i<100;i++){
viewer.entities.add({
    polygon : {
        hierarchy : Cesium.Cartesian3.fromDegreesArray([-115.0, 37.0 + i,
                                                        -115.0, 32.0 + i,
                                                        -107.0, 33.0 + i,
                                                        -102.0, 31.0 + i,
                                                        -102.0, 35.0 + i]),
        material : Cesium.Color.GREEN
    }
});
}

redPolygon.show = false;
setTimeout(function(){
    redPolygon.show = true;
},10000);

viewer.zoomTo(viewer.entities);

which I was just about to open an issue for, so thanks again @cleroux !