Fix polyline collection update when model matrix changes

[hpinkos]

Polyline wasn't updating properly when the PolylineCollection model matrix values changed because it was copying the reference to PolylineCollection.modelMatrix instead of copying the values.

Here is an example that doesn't update in master but updates in this branch.

var viewer = new Cesium.Viewer('cesiumContainer');
var localPolylines = viewer.scene.primitives.add(new Cesium.PolylineCollection());

var lon = -125;
var lat = 35;
var center = new Cesium.Cartesian3();

var localPolyline = localPolylines.add({
    positions : [
        new Cesium.Cartesian3(0.0, 0.0, 0.0),
        new Cesium.Cartesian3(1000000.0, 0.0, 0.0)
    ],
    width : 10.0,
    material : Cesium.Material.fromType(Cesium.Material.PolylineArrowType)
});

setInterval(function() {
    lon += 0.3;
    center = Cesium.Cartesian3.fromDegrees(lon, lat, 0, undefined, center);
    localPolylines.modelMatrix = Cesium.Transforms.eastNorthUpToFixedFrame(center, undefined, localPolylines.modelMatrix);
}, 100);

[bagnell]

Can you add a test and update CHANGES.md?

[bagnell]

Your example doesn't work in master, but its strange that changing the x coordinate of the second point to 1e7 makes it work.

[hpinkos]

@bagnell ready. When writing my specs I also found a spot in PolylinePipeline where I needed to normalize a Plane normal