When a scene is using occlusion culling and you try to use the visualisation mode with the overdraw mode in the scene view, the scene is rendered normally.

To reproduce:
1. Open the attached project(OcclusionOverdraw.zip)
2. Open the "test" scene
3. Toggle the Occlusion Culling in the Visualisation tab while the scene mode is in overdraw mode

Expected result: Overdraw mode should be active on both non-occlusion vis and occlusion vis
Actual result: Overdraw only works when not in occlusion vis mode

Reproduced with: 2017.2.2p3, 2017.4.1f1, 2018.1.0f1, 2018.2.0b1