When "Portal" is enabled in the Occlusion Culling preview menu in scene view, it displays glitchy-looking triangles.

Repro steps:
1. Open TestScene from attached project.
2. Activate Occlusion Window, select Main Camera and go to Scene view.
3. In Scene view, enable "Portal"
4. Move Main Camera in the scene view and observe artefacts.

Expected outcome: portals view to only show Occlusion Portals (if added to scene) or else display useful/intuitive information as to what is being shown.

Notes:
- Not a regression, same behaviour in 5.2.3f1
- Same behaviour on Windows 10
- The preview window itself could be a bit more intuitive. Visibility lines are color-coded nicely (green icon - green lines), but camera volumes has a white icon and yellow bounding box and portals have a gray icon and blue/green visualization in scene (image attached).