Overlapping billboards blended incorrectly

The icon blending issues fixed in #5066 can still be reproduced with a lot of overlapping icons.

Take the following Sandcastle snippet:

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

var altitude = 75000;
var count = 5000;
var opacity = 1.0;
var degrees = 1;

for (var i = 0; i < count; i++) {
    viewer.entities.add({
        position : Cesium.Cartesian3.fromDegrees(Math.random() * degrees, Math.random() * degrees),
        billboard :{
            image : 'http://maps.google.com/mapfiles/kml/shapes/parks.png',
            scale : 0.5,
            color : new Cesium.Color(1.0, 1.0, 1.0, opacity)
        }
    });
}

viewer.camera.flyTo({
    destination : Cesium.Cartesian3.fromDegrees(degrees / 2, degrees / 2, altitude)
});

At the provided altitude, icon borders bleed into other icons:

If you zoom out, the icon borders no longer overlap.

[hpinkos]

Appears fixed

[ZacLiveEarth]

Pretty much any png with anti-aliased edges will do. The tree icon above, for example, should work just fine.