API to quickly check for OSVR server

[JeroMiya]

Unreal issue for reference:
OSVR/OSVR-Unreal#102

Basically, it is desirable to have a way to quickly check whether or not to attempt client context creation (with or without server auto-start). Currently, the client has to try to initialize a client context and wait for a status check. With server auto-start, the wait may be up to 5-10 seconds. In game engines like Unreal that support multiple VR plugins, this means that the OSVR plugin could potentially add 5-10 seconds of delay to game startup even without the user having OSVR installed. This creates an incentive for developers to disable the plugin altogether.

There would be a separate API that lets the client query whether server auto-start is possible on the current platform. For example, on Windows, that query would return true if the OSVR_SERVER_ROOT environment variable was set. On Android it might always return true.

So this would be the typical client workflow with the new API (assuming separate function call for checking if auto-start is possible and checking if the server is running):

• if client wants to attempt server auto start and the API indicates that it's possible
  • attempt it, and then attempt client initialization.
• else if API determines that an OSVR server is running
  • attempt client initialization.
• else do not attempt client initialization.