Steps to reproduce
1) Open the attached project, which sets a custom projection matrix using Camera.SetStereoProjectionMatrix()
2) Set XR settings to VR Supported, multipass
3) Play in editor with either OpenVR+Vive, OpenVR+Rift, or Oculus+Rift

Expected Result:
The custom projection matrix is stereoscopically coherent

Observed Result:
The custom projection matrix is not coherent, resulting in a "double vision" effect.

Observed on 2018.1.7f1, 2018.2.0f1, and trunk (2018 July 2 build).
Issue is NOT present in 2017.4
Issue bisected in 2018.1. Regression occurred between b4 and b5.