Enable user to disable `mesh_show_backface` with new O3DVisualizer

**Is your feature request related to a problem? Please describe.**
The old [`draw_geometries` function](http://www.open3d.org/docs/release/python_api/open3d.visualization.draw_geometries.html#open3d-visualization-draw-geometries) enables the user to toggle whether to show the backface of the mesh. I've been unable to find a corresponding option for the [`O3DVisualizer`](http://www.open3d.org/docs/release/python_api/open3d.visualization.O3DVisualizer.html)

**Describe the solution you'd like**
The same ability to toggle whether to show or cull backfaces of a mesh.

**Describe alternatives you've considered**
I've searched through all the gui, rendering, shader, and materials options, but there doesn't seem to be a way to do this using v0.12.0 through the Python interface.

**Additional context**
My request is based on Open3D v.0.12.0 installed via pip for Linux.