Translucent rectangles very slow when switching to 2D

[likangning93]

Rendering several stacked translucent rectangles seems to severely decrease scene performance, but only when switching to 2D from other scene modes - starting the scene in 2D doesn't seem to have the same performance drop.

I'm not seeing the same performance drop in 1.49 as in master (42 -> 30 FPS vs. 42 -> 14 FPS), so this seems like a recent regression as opposed to a bug we never found before.

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

viewer.scene.debugShowFramesPerSecond = true;

// Tropics of Cancer and Capricorn
var coffeeBeltRectangle = Cesium.Rectangle.fromDegrees(-180, -23.43687, 180, 23.43687);

// Add rectangles to show bounds
var rectangles = [];

for (var i = 0; i < 10; i++) {
    rectangles.push(viewer.entities.add({
        rectangle : {
            coordinates : coffeeBeltRectangle,
            material : Cesium.Color.RED.withAlpha(0.1),
            height : i * 5000.0
        }
    }));
}

Sandcastle.addToolbarButton('Show/Hide Rectangles', function() {
    var rectanglesLength = rectangles.length;
    for (var i = 0; i < rectanglesLength; i++) {
        var rectangleEntity = rectangles[i];
        rectangleEntity.show = !rectangleEntity.show;
    }
});

[hpinkos]

Related? #5026

[lilleyse]

git bisect points to 4633ea6. I'll look into this.