Viewport rects are handled incorrectly in non-stereo cameras when in VR mode, resulting in rendering artifacts.

Reproduction steps:
1. Open the attached project ("UOIAssets.zip")
2. Open the "MRC" scene
3. Enter Play mode
4. Select "OculusMRC_ForgroundCamera" Game object
5. In the Inspector window change Viewport Rect X value

Expected result: The Game window shows the greenscreen along with the foreground assets
Actual result: The Game window shows a black screen or a copy of the background image (see "ZtpNzVWb6D.mp4" video)

Reproducible with: Oculus XR 1.7.0 (2019.4.20f1, 2020.2.4f1, 2021.1.0b7, 2021.2.0a4)
Could not test with: 2018.4.32f1 (XR SDK not available)

Tested:
- Oculus Rift S
- MockHMD