OrientationWidget would sometimes get occluded by data.

OrientationWidget in 3D view would sometimes get occluded by data. That was
because the orientation widget wasn't clearing the zbuffer before rendering the
widget. This fixes that issue.

Change-Id: I34acf612e3641605aafc642101ddf1ba54c2e055
Merge topic 'fix_orientation_widget' into next

c4dbc6b OrientationWidget would sometimes get occluded by data.