Need a function to return extent of a 3D view as we draw to it.

[ianneilmacleod]

Currently making a best-effort to track extent as one draws 3d stuff to a view, but doing this in Python is inefficient. The underlying library should so this.

See related TODO in view.py

[ianneilmacleod]

Linked gxapi issue: https://github.com/GeosoftInc/gxapi/issues/11