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.
Remove picking example
Fix trianglePlaneIntersection doc
Merge pull request #1695 from AnalyticalGraphicsInc/pickingExample

Remove picking example
Merge pull request #1696 from AnalyticalGraphicsInc/docfix

Fix trianglePlaneIntersection doc
Use Cartesian3.fromDegrees and friends in reference doc
Merge remote-tracking branch 'origin/master' into docfix2
Doc fixes
Fix comment.
Merge pull request #1697 from AnalyticalGraphicsInc/docfix2

Use Cartesian3.fromDegrees in reference doc
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
Merge branch 'master' into gltf-next