Fix new billboards added in 2D/CV

New billboards were not added to the `_billboardsToUpdate` array, which meant that `recomputeActualPositions` would not initially update them for CV or 2D mode.  This fixes #1691 by always adding new billboards to the `_billboardsToUpdate` array.
Changes after review

Rather than adding new billboards to billboardsToUpdate, always recompute actual positions for all billboards whenever createVertexArray is true.
Merge pull request #1693 from AnalyticalGraphicsInc/issue1691

Fix new billboards added in 2D/CV