Are we sure this is the right fix?  I think it is masking a bug elsewhere.

Given that `_createVertexArray ` is set above, in `update`, `createVertexArray` should be true and the entire buffer should be written.  I suspect that still happens and then on the second frame, this billboard is updated again masking the problem.

This isn't the full fix, but I think Line [1117](https://github.com/AnalyticalGraphicsInc/cesium/blob/issue1691/Source/Scene/BillboardCollection.js#L1117) should also have `this._billboardsToUpdate = [];` like `removeAll` since no billboards need to be updated after the entire buffer is rewritten.