Workaround needed for render info collection before OpenDisplay

[JeroMiya]

There is a bug in the current implementation when prediction is turned on in the server config. When this happens, the RenderManager code in the render info collection API assumes the display is already open, and fails if it is not. We use this API to calculate the render target sizes, but we need to do so on the game thread, and before the display is opened (which happens later).

The real fix is to implement this issue in OSVR-Core and use the new API instead of RenderManager to calculate those render target sizes:
OSVR/OSVR-Core#316

Until then, we need to add another workaround in OSVR-Unreal that calculates render target sizes without the render info collection API. We could go back to using the DisplayConfig API to get the raw target sizes and then use our existing code that parses the render manager config to adjust them ourselves for oversample and overfill.