Billboard clipped on right side

In Cesium 1.31

I have a large number (500+) of quite large (200x100pixel) billboards on my map, and this problem only occurs when the number of billboards is large. The billboard image are canvases that I draw with javascript within the application.

For some reason, the billboards are clipped on the right hand side:

This can be compared to using the same canvas images as icons in Open Layers:

What is really strange, is that I can almost solve the problem if I set the imageSubRegion to be bigger than the canvas itself.

```js
var entity = {
position: Cesium.Cartesian3.fromDegrees(feature.geometry.coordinates[0], feature.geometry.coordinates[1]),
billboard: {
horizontalOrigin : Cesium.HorizontalOrigin.LEFT,
verticalOrigin : Cesium.VerticalOrigin.TOP,
image: ctx,
imageSubRegion: new Cesium.BoundingRectangle(0, 0, ctx.width+2, ctx.height+2),
height: milsymbol.getSize().height,
width: milsymbol.getSize().width,
pixelOffset : new Cesium.Cartesian2(-milsymbol.getAnchor().x, -milsymbol.getAnchor().y)
}
}
```
Now this will be displayed:

*Better but not perfect*

If we zoom out a lot, we can also see that some of the missing lines from the right hand side is rendered on the left hand side of some symbols:

*Notice the vertical lines that shouldn't be there*

I figured out that Cesium stores the images in some kind of atlas, but I couldn't find where in the code this was done, and what could be the reason for this error. Please let me know if you need more information about this.

Added:
*I tried to revert to Cesium 1.27 to see if #4675 introduced in 1.28 messed anything up, but the result in 1.27 was almost the same as in 1.31.*

Reproduce the report with 500+ large canvas billboards using the shown imageSubRegion, horizontalOrigin, verticalOrigin, and pixelOffset settings in Cesium 1.31. Start by tracing how billboard canvas images enter the image atlas; done means the full right side renders without clipped or duplicated lines, including when zoomed out.

Written by the indexing model from the issue text.