Debugging aids

Lots of goodness for debugging  rendering code for #1025.  From CHANGES.md:

* Added `debugShowBoundingVolume`, for debugging primitive rendering, to `Primitive`, `Polygon`, `ExtentPrimitive`, `EllipsoidPrimitive`, `BillboardCollection`, `LabelCollection`, and `PolylineCollection`.
* Added `DebugModelMatrixPrimitive` for debugging primitive's `modelMatrix`.
* Added `Primitive.allowPicking` to save memory when picking is not needed.

Despite the branch name, point geometry is not included.