CVRResources/IVRResources not exposed in opervr_api.cs

[fraserkeith]

The CVRResources/IVRResources API is missing from the C# API header file opervr_api.cs.

The code is there for it, but it's not exposed as-per the other parts of the API. The documentation implies that it should be as this API and it's methods are listed in the spec.

I'll issue a PR for this and a related bug shortly.