UE-30444 - OSVR is affected by screen percentage Post Process volumes

[JeroMiya]

1. Set up OSVR
2. Start OSVR_Server
3. Run Ue4Editor.exe QAGAME QA­PostProcessing
4. Play in VR­Preview
5. Run to example 1.15, move into the yellow marked zone.
   Result: Begins rendering small in corner, rest of screen ceases to update.

notes:

Screen percentage post process volumes affect rendering on the OSVR. It also renders incorrectly when affected. The incorrect effect has been logged as UE­33363

my comments: We are already handling r.ScreenPercentage correctly. I had previously tried getting r.ScreenPercentage every time it's used (which we still need to do), but that didn't solve this problem. When in the post-process volume, Unreal renders to a small corner of the render target. The only thing we can do is modify our source viewports, but as yet I don't know how we determine when we're in a screen percentage post process volume, and what the screen percentage for that volume is.