The Ambient Occlusion effect within the PostProcessing Stack (V2) shows visual artifacts when changing the XRSettings renderViewportScale or eyeTextureResolutionScale.

Reproduction Steps:
1. Download the smaller project (see comments, below)
2. Power-on your VR headset
3. Build & Run, and experiment with the options:
- Press Fire1 to change renderViewportScale
- Press Fire2 to change eyeTextureResolutionScale

Observations:
Ambient Occlusion shows 'ghosting' issues with renderViewportScale changes, and causes big artifacts when eyeTextureResolutionScale is changed. This primarily happens under the Multi Scale Volumetric Obscurance mode, rather than Scalable Ambient Obscurance.

Reproducible in versions 2018.1.3f1, 2018.2.0b6.